A Special Agent of the Federal Bureau of Investigation, he was the lead agent on Raymond Reddington case until Elizabeth Keen was specifically requested by Reddington during the process of cooperating with Assistant Director Harold Cooper.Ressler is a veteran field agent who has been tracking Raymond Reddington for 5 years, the bulk of his career.He is perhaps more troubled by Reddington’s obsession with Elizabeth Keen, a novice FBI agent and someone Ressler doesn't trust when she is appointed to his case.Ressler is adamant that Reddington should communicate with him and not Elizabeth despite Reddington's insistence on ignoring him.
At the cabin, he leads the raid and calls a medic for Elizabeth.
When Reddington and Elizabeth are in the restaurant he is in the control vehicle monitoring their conversation and the other customers.
When Reddington goes missing, he rushes into the restaurant and attempts to pursue him.
He was once engaged to a woman named Audrey Bidwell, but his obsession with Reddington caused them to break up.
When Reddington turns himself in, Ressler is caught off guard and is unsure of the criminal's motives.
After Elizabeth is in the ambulance, he asks Red about Lorca.