Supervisory Special Agent (SSA) Aaron Hotchner (known to friends and colleagues as 'Hotch') is Chief of the Behavioral Analysis Unit based at the FBI headquarters in Quantico, Virginia. His direct superior is Section Chief Erin Strauss - a woman who will do her utmost to prevent Hotch from rising through the FBI's ranks. He is clearly an ambitious and very able Agent, but has had to forego any real hopes of gaining promotion, because of a stalwart sense of loyalty to his team.

Although Hotch's timeline on Criminal Minds is messy (we are never sure of his age, and calculations of his graduation and other dates sometimes seem inconsistent and implausible), but we know some bits and pieces about him that are 'canonically' correct:

• His birthday is November 2nd - the same day as El Día de los Muertos, the Mexican 'Day of the Dead'. We know this because he had to go to Mexico for a case on his birthday ('Machismo', 1x19).

• He is 43 years old (as seen in his medical records, in 5x01, 'Nameless, Faceless').

• He trained and practiced as a lawyer before joining the BAU. He believed that he could maker a bigger difference if he was actively involved in catching criminals.

• He has a younger brother, Seán, who dropped out of law school to become a chef. Seán appears in 1x16, 'The Tribe'.

• He was married to Haley, who asked for an uncontested divorce in season 3. Hotch signed the divorce papers in 3x14, 'Damaged''. Haley had been unhappy with the fact that Hotch was never at home, or when he was, he could be called away at any moment. In 3x02 'In Birth and Death', she left Hotch, and earlier scenes in the episode implied that she may have been having an affair.

• He has a son, Jack Hotchner, who was born in season 1, making him about four years old. Hotch struggles, from Jack's birth on, to maintain a sensible work/life balance, which he finds almost impossible, given the heavy demands of his job. Cases involving children seem to affect him particularly, as they remind him of his own family.

• His father was a lawyer, and Hotch also indicated in 2x19  'Ashes and Dust', that he was a philanderer, and died from lung cancer when Hotch was quite young. We also know that he had a heart attack when he was around the same age Hotch is now.

• His mother is from Manassas, Virginia, and attended Mary Baldwin College in Staunton, VA (1x11, 'Blood Hungry').
  

Hotch had worked with Jason Gideon and David Rossi when he initially joined the BAU. Aside from those two, he has worked with Derek Morgan the longest. Emily Prentiss was the last Agent to join his team, but he has actually known her for some time before that, although their meeting in 2x09 'The Last Word' suggests that they hadn't seen each other for over a decade since they first met, when Hotch worked with Prentiss's mother, an Ambassador.

In the season 2 episode 'Revelations', Hotch asks the team to tell him what his worst traits are. JJ tells him he's a bully, Morgan calls him a drill sergeant, and Prentiss says that he trusts men more than women. But all conclude that he would never put himself before the team.

Hotch's first case with the BAU was the case of the Boston Reaper, in 1998. The Reaper was profiled as an 'Omnivore', because the victimology and other characteristics relating to the series of murders perpetrated by him were inconsistent. The Reaper disappeared after one of the detectives working on the case with Hotch brokered a deal with him, vowing to stop looking for him, if the Reaper, in return, pledged to stop killing. The deal expired when the detective died some years later.

The Reaper, following his arrest and subsequent escape from prison, ambushed Hotch at his apartment in 4x26. The full extent of what happened is not yet known, other than the fact that Hotch was attacked, and inflicted with nine stab wounds that strategically missed his vital organs. The Reaper used Morgan's stolen FBI credentials to leave Hotch at St Sebastian's hospital. 

Following fears about Jack and Haley's safety, they were taken into protective custody, and brought to a location unknown even to Hotch. In 1x01 'Nameless, Faceless', Hotch expressed his fears that he might not see Jack for years. 

Hotch takes sick leave for a month, and returns to work, behaving in an erratic manner that causes Morgan to confide in Rossi that he is concerned about the Unit Chief's ability to do his job effectively (5x02; 'Haunted')

In 5x05 ('Cradle to Grave'), Section Chief Strauss speaks to Hotch in his office, but we are not privy to the details of the conversation. At the end of the episode, he asks Morgan to step in as Unit Chief for the foreseeable future, telling him that this is the only way he can keep the team together. Morgan hesitantly agrees to take on the role, asking for Hotch's assurance that it will only be a temporary arrangement.