Andrew Hacker is an FBI Assistant Director and Booth's boss.
He was involved romantically with Dr. Brennan.
In The Doctor in the Photo, his name is seen on Booth's phone when he gets a call, suggesting that Hacker is still in charge of Booth, though he hasn't made any physical appearances.
Hacker frequently tells Booth that he, Booth, is by far the better FBI agent of the two of them, and sometimes even the better man. Hacker, however, prides himself on his political skills and instincts, which he often acknowledges are what got him to where he is.
He has a Stratocaster in his office.