- "Zack Addy. I live to serve."
Dr. Zackary Uriah "Zack" Addy is a professional forensic anthropologist who worked for the Jeffersonian Institute. He was originally Dr. Brennan's brilliant young assistant at the beginning of the series before he received his doctorate in Forensic Anthropology in the series' second season.
Coming from a large Michigan family (with three brothers and four sisters), Zack is a former child prodigy, a genius with an IQ well above 163, and a supposed photographic memory. He has started two doctorates, one in Forensic Anthropology, which he has completed, and one in Engineering, which was mentioned in "The Woman in the Car."
His specialty, like Dr. Brennan, is in the analysis of remains, especially identifying the cause of death and weapons from marks remaining on skeletal remains. It is usually his task to remove the flesh from the bones, a process known as debriding. Because of his tremendous intellect, he has a strong broad-based knowledge of many of the specialties in the Jeffersonian lab. Despite his intelligence, Zack is unsure of himself and though he has come up with crucial insights vital to some of the team's cases, he is unable to forcefully express his opinion to Dr. Brennan. This may be due to hero worship and the feelings he might have toward her.
Zack's best friend seems to be Jack Hodgins. Although it at first appears Hodgins and Zack are roommates, it is later revealed Zack rents the apartment over Hodgins's garage. Zack also carpools with Hodgins because he neither drives nor rides a bike as was revealed in The Man on Death Row.
Episodes towards the end of Season 1 of Bones reveal Zack's colleagues, especially Dr. Goodman, feel he has become too comfortable as Dr. Brennan's assistant. He's therefore not completing any of his work towards his doctorate degrees to avoid having to move into a new position. Goodman and Hodgins conspire to make Zack less comfortable in his position to motivate him to complete his studies and assume a role above that of an assistant.
Right before he was about to complete his doctorate, Zack asks Dr. Saroyan if he could have a job working at the Jeffersonian. She replies she could not put him in front of a court to testify because people would not take him seriously. Zack then goes to ask Angela for fashion advice, and she gives him a complete makeover that includes a new haircut and suit. After completing his doctorate and getting the makeover from Angela, Zack again asks Dr. Camille Saroyan for a job, and she gives it to him.
At the end of Season 2, Zack receives a request from the office of the President to ship out to Iraq. What his duties there would be are not revealed, and he only tells Hodgins and Booth about it. He initially seems inclined to go, but after talking about the reality of war with Booth, he seems less sure, asking Booth for advice because he "knows more about duty and honor than anyone else" he knows. He also turns down the offer to be the best man at Hodgins and Angela's wedding in case he decides to go to Iraq and is killed. This is because he doesn't want Hodgins's memories of the wedding to be tainted with sadness. It is revealed in the first episode of Season 3 Zack had just returned from a three-month stint in Iraq.
In "The Pain in the Heart," the final episode of Season 3, Zack receives third-degree burns and massive tissue damage (the cartilage on his left hand was destroyed, among other things) on both hands after an explosion in the lab. It's later revealed he was working as the apprentice of The Gormogon and the explosion was designed as a distraction so the Gormogon could break into the lab and steal a skeleton. Zack's weaker personality was manipulated by the Gormogon (whom he referred to as "the Master") so completely Zack believed his belief system irrefutable. He even went so far as to kill a lobbyist and dividing his bones among the Jeffersonian's storage units. However, Zack still maintained a loyalty to his friends, willing to injure himself to keep Hodgins safe, and giving up the location of Gormogon's house.
At the end of the episode, prosecutor Caroline Julian says while Zack has confessed to killing Ray Porter; she's willing to work out a deal to have him declared non-compos mentis, a decision committing him to an asylum rather than prison. Zack will no longer be a regular character on the show. However, series creator Hart Hanson said he may become a recurring character to provide consults to the team with "certain talents we can use in a 'Hannibal Lecter' kind of way." 
The episode "The Perfect Pieces in the Purple Pond" reveals Zack is receiving psychological treatment from Sweets. It's also shown at least Hodgins and Angela have both been in contact with Zack during his imprisonment. Hodgins is shown bringing Zack a mathematical riddle to solve, which he solves in less than 30 seconds. In his session with Sweets, Zack admits to feeling guilty for killing the lobbyist, but argued he committed the crime for what had appeared to be perfectly logical reasons at the time. "I was wrong, not delusional," he says. Sweets believes Zack should feel more guilty about killing the lobbyist and less about having been taken in by Gormogon's rhetoric.
Towards the end of the episode, Zack escapes from the institution. It's revealed he could have escaped at any time, but didn't feel it was necessary to do so until that particular moment. After helping his former coworkers solve the case, Zack willingly returns to the institution with Sweets and Booth.
After Booth relinquishes Zack to Sweets' hands, Zack tells Sweets that while he had helped Gormogon find the lobbyist, he himself hadn't actually stabbed Ray Porter. In other words, Zack believes he killed the lobbyist, but from a legal standpoint, he's considered only an accessory or co-conspirator to murder. Sweets insist Zack change his story, but Zack refuses. Zack fears if his secret were to come out, he will find himself in prison, where he is assured, by Hodgins, to do very poorly at. Sweets wanted him to tell the others the truth since he believed that The Apprentice is still at large. Zack reassured Sweets that the apprentice who murdered Ray Porter was killed by The Master so he could recruit Zack because "there could only ever be two." He reminds Sweets as Zack's therapist, he must not reveal Zack's secret because if Sweets were to do so, he would be violating doctor-patient confidentiality. The episode closes with Zack in the Psychiatric Hospital and Sweets keeping Zack's secret, albeit reluctantly.
In the season 4 finale, Zack is considered a suspect when a man is murdered in a popular nightclub owned by Booth and Dr. Brennan. Zack is described as a "moron who goes to jail for a murder he didn't commit" by Vincent, suggesting Booth (at least subconsciously) knows Zack is innocent. At the end of the episode, we learn the entire thing was a dream Booth had while unconscious and part of a new book Brennan was writing.
Zack also returns as a guest star in the season 5 episode The Parts in the Sum of the Whole. We see him in a flashback as Dr. Brennan's graduate student while she works on her first case with Booth.
After being framed as the Puppeteer, Zack recants his confession in the murder of Ray Porter, having realized that he could never take a life, even in self-defense. The Jeffersonian team set out to exonerate Zack through season 12, eventually finding the Apprentice's body with the help of Doctor Gordon Wyatt. Using blood evidence from the Apprentice's body, Zack was exonerated of Ray Porter's murder at his appeal in The Day in the Life and his life sentence overturned. However, his conviction for aiding a known killer still stands and he will have to serve out the remainder of his sentence for that crime, a mere thirteen months. Once that time is up, Zack will be released into society once more.
- "Booth: You're a genius... who can't drive?
Zack: If you knew what I knew about Structural Design, you wouldn't drive either."
- ―The Man on Death Row
Despite his intelligence, Zack is unsure of himself and though he has come up with crucial insights vital to some of the team's cases, he is unable to forcefully express his opinion to Dr. Brennan. This may be due to a hero-worship and also to romantic feelings toward her. When he discovered Temperance's own Forensic Anthropology professor had become her lover, Zack repeatedly wondered aloud whether he might enter into a similar relationship with Dr. Brennan. He was quickly disabused of the notion by his colleagues.
Zack appears to have an on and off-again relationship with "Naomi in Paleontology," despite hints in the first season she was dissatisfied with his sexual prowess. In the third season, they accompany each other to the annual Jeffersonian Institute Halloween party, agreeing to dress as the front and back half of a cow ("Mummy in the Maze"). Zack has no problem making inappropriate comments about others' personal lives and asked Agent Booth for advice on sex and women, requests which Booth characteristically ignored. Angela answered his question once, and Hodgins gave him a book (Kama Sutra) to help him out ("The Pain in the Heart").
Very little is known about his childhood. In "The Wannabe in the Weeds" episode from the third season, it was discovered Zack was a singer during his childhood as a way for his parents to help integrate him socially. Though it did not appear to work, he demonstrates his talent when Hodgins doubted him. He has fond memories of receiving his first microscope ("The Girl with the Curl") and when he was six, he had a pirate eyepatch ("The Man with the Bone"). It is also known he attended a private high school ("A Boy in the Tree"). His interests include model airplanes ("The Killer in the Concrete"), watching basketball ("The Soldier on the Grave"), and science fiction: Firefly ("The Man in the Fallout Shelter"), Star Wars, Star Trek, Stargate, Battlestar Galactica ("The Superhero in the Alley"); he also likes Lord of the Rings ("The Woman at the Airport"). He does not dance, because he's been told he "look[s] like a marionette in a windstorm" (The Man in the Wall).
According to Emily Deschanel, Zack "almost definitely has Asperger's syndrome," a condition on the autism spectrum. Although well-meaning, helpful, and friendly, when a situation calls for social interaction or intuition, he is often lost. Further evidence of his social ineptitude can be seen in the frequent, on-screen coaching in social matters he gets from Jack and Angela. He is very literal and is often confused by colloquial expressions or metaphors, despite his high intelligence. His attempts to use such expressions meet with mixed success, such as referring to a skull he'd cleaned as being "clean enough to eat off of." ("Two Bodies in the Lab")
In the episode "The Killer in the Concrete", Booth is on the phone with Zack and Dr. Brennan while looking for "Icepick" (real name, Hugh Kennedy) at a model airplane enthusiasts gathering. Unaware of Zack's interest in planes, Booth comments every "airplane freak" in the area was at the event, and Zack corrects him by saying the enthusiasts prefer to be called "pilots." Zack mentions forensic anthropology is only one of his doctorates, the other is in applied engineering, and he is extremely adept at practical aeronautics.
As seen in the episode "The Man in the Fallout Shelter" Zack has a large family and greatly values them, remarking the true meaning of Christmas is "Brothers, sisters, nieces, nephews; 40 people who love you and are happy to see you." Also in the first season, he reveals to Dr. Brennan he uses his vacation time to visit his family. While he does not necessarily enjoy these visits, he goes because they love him ("The Man in the Wall").
In "The Pain in the Heart", Zack's care for his friends is shown: even when working for the Gormogon he protects Hodgins when he knew he'd seriously injure himself in the process. Looking through his favorite things later, his friends discover that they were all things they'd given Zack which Sweets believes is significant.
Episodes towards the end of the first season of Bones reveal that Zack's colleagues, especially Dr. Goodman, feel he has become too comfortable as Dr. Brennan's assistant. He's therefore not completing any of his Doctorates to avoid having to grow into a new position. Goodman and Hodgins conspire to make Zack less comfortable in his position to motivate him to complete his studies and assume a role above that of an assistant.
Right before he was about to complete his doctorate, Zack asks Dr. Saroyan if he could have a job working at the Jeffersonian, but she replies that she could not put him in front of a court to testify because people would not take him seriously. Zack then goes to ask Angela for fashion advice and she completely redoes his look including a new haircut and suit. After completing his doctorate, and getting a makeover from Angela, Zack again asks Dr. Camille Saroyan for a job, and she gives it to him.
At the end of the second season, Zack receives a request from the office of the President to ship out to Iraq. What his duties there would be is not revealed, and he only tells Hodgins and Booth about it. He initially seems inclined to go, but after talking about the reality of war with Booth, he seems less sure, asking Booth for advice because he "knows more about duty and honor than anyone else" he knows. He also turns down the offer to be the best man at Hodgins and Angela's wedding in case he decides to go to Iraq and is killed because he doesn't want Hodgins' memories of the wedding to be tainted with sadness. In the first episode of season 3, it is revealed that he did in fact go to Iraq but returned in that same episode.
In the final episode of season three, The Pain in the Heart, Zack receives third-degree burns and massive tissue damage (the cartilage on his left hand was destroyed, among other things) on both hands after an explosion in the lab. It is later revealed that he is working as the apprentice of the The Gormogon serial killer, and the explosion was intended as a distraction so Gormogon could break into the lab and steal the silver skeleton. Zack's weaker personality was manipulated by the Gormogon (whom he referred to as "the Master") so completely that Zack believed his belief system irrefutable, even killing a lobbyist and dividing his bones among the Jeffersonian's storage units. However, he still maintained a loyalty to his friends, willing to injure himself to keep Hodgins safe and giving up the location of Gormogon's house after Brennan refuted his logic.
At the end of the episode, the prosecutor Caroline Julian says that while Zack has confessed to killing the lobbyist she is willing to work out a deal to have him declared non-compos mentis, a decision which will commit him to an asylum rather than prison. Zack will no longer be a regular character on the show, but series creator Hart Hanson said that he may become a recurring character to provide consults to the team with "certain talents we can use in a 'Hannibal Lecter' kind of way."  As revealed in later episodes, Zack got a life sentence for the murder of Ray Porter and a separate sentence of around ten years for aiding a known killer for his work as the Gormogon's apprentice.
Zack begins to receive psychological treatment from Sweets (The Perfect Pieces in the Purple Pond). Feeling guilt about aiding a serial killer and admitting to his mistake, Sweets believes that Zack shouldn't feel bad about being wrong, but about being delusional during the period of being Gormogon's apprentice. Not only that, Sweets believes that Zack should actually feel guilty about killing the lobbyist more than about being wrong.
Zack later escapes the institution by swapping the magnetic strips from Sweets' key card to Zack's library card. After solving the case, Zack realizes it's time to go back to the institution where Sweets was waiting. After Booth relinquishes Zack to Sweets' hands, Zack admits to not killing Ray Porter and that the previous apprentice had killed the lobbyist instead. Since the Gormogon can only have one apprentice at a time, he killed the original lobbyist's murderer in order to take Zack under his wing. Though not the actual killer, Zack does not want the truth to be revealed to his colleagues at the Jeffersonian because if his secret were to come out, Zack would find himself in prison since he is still an "accessory" to the lobbyist's murder. Out of fear of finding himself in prison (due to Hodgins' firm belief that Zack "would NOT do well in prison"), he reminds Sweets that, as Zack's therapist, he must not reveal Zack's secret without his permission. If Sweets were to do so, he'd be violating doctor-patient confidentiality. The episode ends with Zack at the sanitarium and Sweets keeping Zack's secret, despite his visible worries about Zack.
In the finale The End in the Beginning he plays the part of Brennan's assistant at the nightclub, which reflects the situation that he and Brennan had in the previous seasons.
Zack's last appearance to date was in The Parts in the Sum of the Whole, which flashbacked the first case Booth and Brennan worked together.
Hodgins mentioned Zack in The Movie in the Making. Pictures of Zack were seen during Hodgins' dialogue. He told them that he was a dear friend and he thought that working with death and murder may have been too much for Zack. The producer, Alex Duffy, thought that working with death was the reason that Zack was institutionalized. Hodgins is seen twisting his wedding ring when he mentioned Zack, possibly thinking about how much he misses him.
In the season finale, it's revealed that Zack is believed to be the latest serial killer that had been living with his victims as living puppets. The Puppeteer's killings started after Dr. Brennan and Booth left the Jeffersonian in The Next in the Last. The subtle hints where shown to Bones in her dreams, Booth was quick to pick on it when he read her psychologist's notes about dreaming of Wendell's "burned" hands. Though they didn't want to believe Zack would do it, Booth quickly went to the institute to find him, only to find his bed vacant; however, the head nurse believes up until the discovery he had not left.
The episode ends with him greeting Dr. Brennan in The Gormogon Vault.
Zack was revealed to be attempting to inject himself with a truth serum to convince Brennan that he never killed anyone, but was arrested by Booth. Dr. Mihir Roshan, Zack's doctor, told them that he was visited almost every week by Sweets and that when he heard that he died, he fell into a fit of rage and devastation and hit himself on the head which left a long scar on his forehead. Angela discovered that Zack was hacking into the E-mails of Brennan, Hodgins, Angela, and Cam, and that he was even posing as Hodgins' doctor and told his physical therapist to perform a procedure to restore Hodgins' ability to walk. It was discovered that the procedure has a less than 1% chance of success, but Zack wanted to give Hodgins hope and was afraid that it only caused him more pain. Karen Delfs believed that the trauma to Zack's head caused him to have Dissociative Identity Disorder and wanted him to review the case file hoping that it will lure out the alternate identity. Zack took a look at the evidence and told Dr. Roshan that he wants to return to McKinley Psychiatric Hospital, but this time to Maximum Security because he and Brennan agreed that the circumstantial evidence against Zack is conclusive.
Brennan discovered that Zack's doctor was The Puppeteer and sent Booth to go after him. Zack, when Roshan was going to administer something into his arm, discovered that the syringe was filled with Succinylcholine which was used to poison the other victims and attacked Roshan and temporarily disoriented him. Zack picked up the syringe and was going to use it against him, but ultimately couldn't kill him. Roshan stabbed Zack's leg with a pen and was going to finish him off, but Booth shoots Roshan in the back, killing him and saving Zack. The next day, Zack confessed that he never killed Ray Porter and the reason that he confessed was that he thought that he would if The Gormogon ordered him to until he learned that he is not capable of killing, not even to save himself. Brennan and Booth agreed to re-examine the evidence and help him get released from the institution.
In The Brain in the Bot, Booth revealed that he got an appeal date set for Zack in a couple of months. The approval letter states that the judge approved the appeal to consider new osteological evidence. Though Brennan has no new evidence yet, Booth expresses faith that she will find it by the time the appeal comes around.
In The Flaw in the Saw, Hodgins finds evidence that can exonerate Zack, but Cam refuses to accept it and accuses Hodgins of planting evidence to free Zack. Though Hodgins tells her to throw it out if she doesn't trust him, Cam is seen going through it once Hodgins leaves. The evidence is ultimately thrown out as later revealed in The Steal in the Wheels.
In The Steal in the Wheels, Zack has two weeks until his appeal and the Jeffersonian team is no closer to finding proof of his innocence. After a failed search by Doctor Gordon Wyatt through Lance Sweets' notes on his sessions with Zack for useful information, Hodgins instead turned his efforts to locating the body of the Apprentice, Ray Porter's true killer so as to examine it for evidence pointing to his guilt instead of Zack's. With the help of Wyatt and Angela, Hodgins is eventually able to locate the Apprentice's body beneath an Acacia Tree in Washington, DC. On the Apprentice's cuff is blood, presumably from Porter, the evidence that can potentially exonerate Zack.
In The Day in the Life, Zack represents himself at his appeal, having studied courtroom procedure for many years. Hodgins provides the new evidence proving that the Apprentice is the lobbyist's killer, but Caroline Julian argues against releasing Zack for the sake of the victim's family, enraging Hodgins. When the time to give closing arguments comes, Zack chooses to let the evidence speak for itself but Brennan speaks up for him, telling the judge of the over 50 killers he helped put in his prison and his efforts to help Hodgins. Based on the new evidence, Zack is exonerated for the murder of Ray Porter and his life sentence overturned. However, as he aided a known killer, that charge still stands and Zack will have to serve out the remaining thirteen months of his sentence for that crime, something he gladly accepts. Brennan also realizes that Caroline was merely doing her job but did so in a way as to ensure Zack's exoneration.
In The End in the End, while going through his things in the ruined lab, Hodgins finds a picture of himself and Zack and smiles at it.
Zack appears to have an on-again, off-again relationship with "Naomi in Paleontology", despite hints in the first season that she was dissatisfied with his sexual prowess. In the third season, they accompany each other to the annual Jeffersonian Institute Halloween party, agreeing to dress as the front and back half of a cow.
Unnamed Performance Artist
In The Girl in Suite 2103, Zack tells Cam that his closest friend outside the lab is a female performance artist.
Zack seems to be friends with Jack Hodgins, with whom it was once thought he was roommates. He actually lives in the apartment above Hodgins' garage on the grounds of his large estate. In more than one episode, Hodgins refers to Zack as his "best friend." He often competes with Hodgins to be "King of the lab", a title claimed by the one who makes a pivotal discovery or conclusion detrimental to a murder investigation. It is often implied that Zack and Hodgins have a brotherly bond.
In The Man in the Bear, Zack competes with Hodgins over a beautiful delivery lady named Toni. Hodgins gave Zack a small copy of the Kama Sutra to get him to stop asking Booth questions about sexual intercourse. In fact, he rents from and also carpools with Hodgins, since he can't drive or ride a bike, simply because he refuses to learn because of what he knows about Structural Design. He once made a comment to Booth that if he (Booth) had the same understanding of structural engineering, he would be afraid to drive as well. In The Pain in the Heart, despite working for the Gormogon and the beliefs he got as a result, Zack protects Hodgins from an explosion he set despite knowing he'd injure himself severely. Bones is later able to use his protecting Hodgins above his own beliefs to get him to turn on the Gormogon. Hodgins is later stunned by the fact that Zack actually listened to all of his conspiracy theories which had an impact on him. In The Perfect Pieces in the Purple Pond, Hodgins is shown to visit Zack in the mental hospital and has hope that one day Zack will be able to rejoin them in the lab. He still refers to Zack as his best friend and while joking with Zack, nearly calls himself "King of the Lab" but stops himself when he remembers Zack is no longer at the lab. He states his hope that Zack will one day return and be "King of the Lab" again. Hodgins also refuses to take Zack's space in the lab which Cam tells him she had suggested as she felt Zack would be most comfortable with Hodgins taking that space. In The Hope in the Horror, it was revealed that Zack was consulting with Hodgins' physical therapist as a world-renowned neurosurgeon named Dr. Alexander Bancroft. He wanted to help Hodgins by having his physical therapist perform a protocol developed by the real Alexander Bancroft in an attempt to help Hodgins recover from being paralyzed, but the procedure has a less than 1% chance of success. Zack wanted to give Hodgins hope since he believed that hope has the power to heal, but his fear is that all he brought him is more pain. In The Flaw in the Saw, Hodgins works to find evidence to exonerate Zack. While he finds something, Cam accuses him of planting the evidence as they all know Zack is innocent and want him free, which infuriates Hodgins. In The Steal in the Wheels, Hodgins is able to locate the body of the Apprentice and as stated in The Day in the Life, developed a protocol where no one would examine the case evidence alone to ensure no accusations of impropriety can be brought against him. During Zack's appeal, Hodgins testifies to his friend's innocence and is enraged by Caroline Julian's apparent efforts to keep Zack locked up, not understanding that she is merely doing her job but in a way that discreetly helps Zack. Hodgins efforts to find proof ultimately result in Zack's exoneration for the murder of Ray Porter, though he must still spend thirteen more months locked up on the charge of aiding a known killer. It has been speculated that due to his close friendship with Hodgins, he would be re-hired at the Jeffersonian after his release.
Earlier in the series, it is suggested that Zack shows sexual affection to Dr. Brennan. While later on in the series in The Pain in the Heart, it's seen that Dr. Brennan and Zack instead have a mother-son bond. Zack hung on to the letter that Brennan sent him when she chose to hire him as her assistant. Zack later breaks out of the mental institution to protect Brennan from The Puppeteer. In The Day in the Life, Brennan speaks for Zack's character at his appeal, something that Brennan herself points out is out of character for her as she only ever testifies to the forensic evidence in a case. Brennan notes to the judge his efforts to cure Hodgins and all the killers he helped put away as evidence that Zack is still a good man who made a terrible mistake. After getting Zack's partial exoneration, the two share a hug with Brennan sad that they couldn't get him released completely. However, Zack is grateful for her efforts as she has ensured that he will only be locked up for a little over a year more rather than the rest of his life.
Zack and Booth hardly ever got along throughout the first three seasons, mostly because Booth threatening to shoot Zack (and Hodgins) on a regular basis whenever he feels that they are annoying him. Despite their unstable working relationship, the two of them maintain a mutual respect towards each other, particularly after they both spent time in Iraq to serve the military. Booth gave Zack a harmonica before he was shipped out to Iraq. Over the course of eight years after the events in the Gormogon case, Booth seems to visit Zack to consult with him on forensic investigations because Booth knows exactly where Zack's room is in the institution as was revealed in The Nightmare in the Nightmare. In The Hope in the Horror, Zack told Aubrey that he and Booth are not friends and thus he did not monitor Booth as he did the rest of the team. Despite this, Booth is shown to believe in Zack's innocence in the murder of Ray Porter and be skeptical of Zack being the Puppeteer. After Zack confesses to his innocence, Booth immediately gives Zack his support in getting exonerated and works as hard as the Jeffersonian team to help Zack in his own way, even moving up the court date for Zack's appeal and providing Dr. Wyatt his session notes with Sweets.
Angela and Zack have a healthy friendship. She gave Zack advice on saving his relationship with Naomi in Paleontology in A Boy in the Tree by telling him to tell her to introduce him to the secrets of love to leave both of them on a clean slate. She even gave Zack a make-over in Judas on a Pole so he can stay at the Jeffersonian after receiving his doctorate. Zack hung onto a caricature she drew of him as King of the Lab. In The Flaw in the Saw, Angela aids Hodgins in trying to find the evidence in Ray Porter's bones to exonerate Zack. With Angela's help, Hodgins is able to find the body of the Apprentice and the needed proof to exonerate Zack after working with Doctor Gordon Wyatt.
Zack and Cam have a mutual friendship outside of work. After working with everyone at the Jeffersonian after accepting what she believes are their faults, Cam eventually accepts Zack as well as everyone else to a point. After she discovered that Zack was working with Gormogon, she started talking about him coldly in the aftermath, but Sweets knew she meant the opposite of what she was saying. Cam once gave Zack a small trophy with a plaque saying: "Zack Addy. King of the Lab." which he waved at Hodgins' face every day. In The Flaw in the Saw, Cam accused Hodgins of planting evidence to free Zack. However, after Hodgins leaves, Cam can't bring herself to throw it out and looks through what Hodgins gave her anyway before throwing it out. Despite this, Cam is shown to be in agreement with the team that Zack is innocent and wishes to see him free as much as anyone.
Sweets became Zack's psychologist after he was institutionalized. According to Doctor Roshan, Sweets visited Zack almost every week for the past seven years. Zack trusted Sweets with his secret, that he never killed Ray Porter while he was working for Gormogon. Sweets, despite his worries about Zack, agreed to keep his secret to avoid violating their doctor-patient confidentiality. Sweets visited Zack every week up until his death. After Sweets was murdered, Zack had trouble processing and he fell into a fit of rage and injured himself on the head, leaving behind a long scar across his forehead and became withdrawn ever since. Despite their professional courtesy, Zack regards Sweets as a friend and most definitely misses him a lot.
Dr. Goodman often treated Zack in a boss-employee situation. He and Hodgins tried to make him feel uncomfortable since Goodman believed that Zack had become too comfortable with his position as Brennan's assistant. Goodman wanted Zack to "grow up" and help him receive his doctorate. Unfortunately, with Daniel Goodman's disappearance, he has never seen Zack achieve his doctorate.
Wendell first met Zack after he escaped from the sanitarium to solve the murder of Jared Addison. He called Zack a "psycho" since he helped Gormogon during his crime spree, but Angela tells him that they all still love him despite the betrayal. When Zack was accused of being The Puppeteer, Wendell immediately had his doubts on whether Zack was actually the serial killer and believed in his innocence knowing that the real killer is highly manipulative. Despite their brief history, Wendell could very well call Zack a friend.
Clark met Zack after he came back from Iraq at the Season 3 Premiere. He was one of 19 interns that Brennan had gone through to replace Zack and prolong her separation from Booth. After Zack came back, Clark had left. They met again in The Verdict in the Story when Zack was working for the prosecution to prove Max Kennan's guilt in the murder of the deputy director of the FBI Robert Kirby. While he was examining the bones, Clark wondered if Zack was screwing with him, which he rejects. Clark was able to find something that Zack missed through a penetrant test, which surprised even him. Clark's personal opinions about Zack are unknown, but it is implied that they seem to display some signs of respect towards each other in a professional courtesy.
Aubrey arrested Zack when Booth found him and Brennan in the Gormogon Vault and interrogated him after meeting with Dr. Roshan. Zack, at first, only wanted to talk to Brennan because he felt that talking to someone of lesser intellect was difficult for him, to which Aubrey takes no offense. He believed Zack to be crazy for kidnapping Brennan, even though Booth expressed that what he tried to do for Hodgins with his paralysis was nice. Aubrey thought he was guilty the entire time and reminiscent of both Norman Bates and Hannibal Lecter. Although Zack was proven innocent of The Puppeteer's crimes, how Aubrey feels about Zack is possibly unchanged due to his awkward behavior.
As the Gormogon's apprentice, Zack worships him and calls the Gormogon "the Master." After having fallen under the Gormogon's sway, Zack was willing to betray his friends to aid the Gormogon and believed himself capable of committing murder on the Gormogon's orders. However, Zack's loyalty to his friends is shown to be stronger than his loyalty to the Gormogon as he was willing to injure himself rather than harm Hodgins, something that would've been against what the Gormogon taught him. After the logic that led to him following the Gormogon was refuted, Zack turned on him completely and helped Booth bring "the Master" down. Zack's relationship with the Gormogon led to him a life sentence in a mental institution for committing murder on the Gormogon's behalf. After his encounter with the Puppeteer, Zack realized that he never would've been able to commit murder for the Gormogon if he had been ordered as he previously believed.
- In The Perfect Pieces in the Purple Pond, Zack is shown to be wearing gloves all the time which Hodgins states are because of the explosion. Presumably, the nerve damage to his hands somehow requires him to need gloves until they completely heal. During the events of that episode, he estimates to Sweets that he regained over 60% full function of his hands which is a best-case scenario for the aftermath of the explosion. It is definite that his hands have long since healed through the progression of the series. However, when Zack returns, he still wears these gloves. It can be assumed that he is required to continue to wear his gloves to help compensate for the lasting nerve damage to his hands that he sustained in the explosion or he is self-conscious in removing his gloves and revealing the scars on his hands to anyone, even himself. At some point between The Perfect Pieces in the Purple Pond and The Hope in the Horror, Zack replaced his black wool mittens with black leather gloves.
- Sweets was the only person who knew that Zack is innocent of the murder of Ray Porter. Since Sweets died in The Conspiracy in the Corpse, what would happen to Zack remained unknown until Season 11 when he escaped the institution.
- Zack is the first main character to return to the show after eight years, the second-longest interval of time that anyone remained unaccounted for before returning. The character who was gone the longest before returning was Tim Sullivan.
- Zack was mentioned to dance like a marionette in a windstorm in The Man in the Wall.
- He said that he had a Michael Jackson glove when he was in school in The Player Under Pressure. In The Woman at the Airport, Angela once asked if the names Michael Jackson or Joan Rivers mean anything to Zack. He responded by telling her that he is familiar with one and that he would look up the other one. It can be assumed that he was familiar with Michael Jackson, but not with Joan Rivers at the time.
- He had an eyepatch when he was six as did many other boys as Hodgins implied in The Man with the Bone.
- Zack revealed that he used to be a singer when he was younger in The Wannabe in the Weeds and he even sang the song; "Love Is A Many Splendored Thing" by Paul Francis Webster and Sammy Fain, to prove it to Hodgins, who, in turn, compared his singing to Italian Opera Singer; Luciano Pavarotti. In real life, Eric Millegan, who plays Zack, studied musical theater at the University of Michigan, where he graduated with a Bachelors Degree in Fine Arts.
- Zack's character can be seen as a facsimile of the Marvel Comic book hero Doctor Strange as both characters were brilliant doctors who lost the mobility in their hands due to a traumatic accident.
- Mitovitch, Matt Webb. "Exclusive: Bones Boss Responds to Finale "Zack-lash"", TV Guide, May 21, 2008.
- "A Boy in the Tree", Bones Season 1 Episode 3.
- Gray, Ellen, Boreanaz says 'Bones' is not procedural, Philadelphia Daily News, January 31, 2007.
- Mitovitch, Matt Webb. "Exclusive: Bones Boss Responds to Finale "Zack-lash"", TV Guide, May 21, 2008.