Karen Delfs is a Criminal Behavioral Analyst from the BRIU. She helped the team analyze the behavior of the suspects they interrogate. She even looked through the files of the Jeffersonian team members to know about the people she works with. At some point, she crossed paths with Caroline Julian and Zack Addy in the past.
Karen: Oh, my God. I just realized I was so focused on this case that I forgot to eat today.
She assisted to help the team locate the killer who set off an explosion that killed multiple cops and Crime Scene Techs who worked at the Jeffersonian. She was shown to be a bit high strung at times and she seems to carry multiple kinds of materials in her purse, including a mouse trap even she unknowingly put in there. Despite her quirks, she is shown to be thorough at her job, even forgetting to eat at times. After the case was solved, she even took Aubrey's burger and took a bite from it when she remembered that she hadn't eaten anything all day.
She returned to give Booth and Aubrey a profile for the latest Serial Murderer; The Puppeteer. She determined that the Puppeteer is extremely intelligent, possibly schizophrenic, and "completely and totally cray cray." After Booth brought George Gibbons in for an interrogation, his lawyer stated that Gibbons is confessing to the murder of Allison Monroe, but Delfs proved that he was only an accomplice to the real killer by determining through his journal, which has over a thousand references of blood and Gibbons deciding not to use his buck knife on Booth when he was in his house, that he suffers from Vasovagal Syncope. Gibbons fainted when Karen sliced the palm of her hand with her pocket knife to prove her theory. She later came in to interrogate Gibbons to get him to reveal the identity of The Puppeteer, but Aubrey told her that Gibbons hanged himself in his cell, leaving the identity of The Puppeteer unknown.
She originally came to ask Aubrey on a coffee date, but he declines because he is dating the intern; Jessica Warren, and he doesn't want to mess it up, which she understands. She went with Aubrey to the office building of the victim, where they find all the blackmail files he hid in a room concealed behind a false wall in the bathroom. Karen found Aubrey's Father's File and insisted that Aubrey should read it, but he refuses to read it because his father, who fled to Croatia, has no relevance to the case. After helping them solve the case, she told Aubrey that she is being transferred to Kansas City the following Monday. She wanted Aubrey to look at his father's file before she left to continue her date who she described to be as "dumb as a box of hair." At first Aubrey declines again, but she stated that the file she found was started six months ago and that Aubrey's father is no longer abroad in Croatia and is currently in Washington D.C. and he is currently investigating Aubrey. She walks away as Aubrey looks through Frank Kwiatowski's file on his father in shock and discovers multiple images of himself and other personal documents.
She returned to her job at Washington D.C. and helped the team attempt to prove that Zack was The Puppeteer. It was later revealed that she knew Zack personally because he offered her a series of interviews at the institution when she was working on her dissertation. After looking at the evidence, she discovered that The Puppeteer suffers from D.I.D. (Dissociative Identity Disorder) and she believed that despite that fact that he doesn't display any symptoms, Zack might have it because of the trauma to his head. She wanted to give Zack the case file because she believes that if Zack is confronted by what he supposedly did then the other personality would present itself. She even had Brennan come into the same room with the two of them to keep pushing Zack. Booth, acting on evidence presented to him by Brandon Faulk, wanted to know why she left her last job at the Kansas City field office. Aubrey talked to Paul Crawford, Karen's former boss from Kansas City, but he told Booth that he refused to answer because he "legally" couldn't comment. Karen tried to help Brennan after Zack was sent back to the institution, but she believed that Karen was the killer and that she planted herself in the case to maintain control. Karen laughed at that idea and told Brennan that the reason she came back to D.C. was because Paul Crawford kept hitting on her incessantly, despite the fact that he was married, so she hit him back... "incessantly". After Brennan and Wendell discovered that the fourth victim they discovered in Graham Reynolds' basement was not a victim, but the exhumed conjoined twin brother of The Puppeteer, Karen deduced that after being separated from his other half, The Puppeteer adopted the dead twin's personality. They were able to discover that The Puppeteer was Dr. Mihir Roshan; Zack's doctor and the Head of McKinley Psychiatric Hospital.
- She does not like pickles. (The Doom in the Boom)
- She studies extensively on the people she works with and the people she interrogates.
- Aubrey describes Karen as a real life Clarice Starling.
- Karen often forgets important things when she focuses on her work, including eating.
- When he was institutionalized, Zack gave her a series of meetings with him when she was working on her dissertation as was revealed in The Hope in the Horror.
- It was widely rumored that she may have been The Puppeteer because of her access to the psychological profiles of the Jeffersonian Forensic Team and the F.B.I. for a time.