Ethan Decker is an agent with the CIA.
Was an agent. That is a very important distinction one. Was. As in, I quit, I'm out of here, don't contact me - ever.
His reason was a very good, and very sad one (no spoiler here) but when we meet him he has been away from the action for some time living largely off the grid in the desert in the western part of the U.S.
Before he quit, he had been one of the very best that the CIA's Strategic Counter-Terrorism Center had going for it. He had been a member of the prestigious Delta Force before Avery Cox of the Agency found and recruited him. Though Cox was a particularly loathsome man, he appealed perfected to Decker's inate need and love of service to others.
Though Decker had had the need to take lives in his career, especially with Delta, at the SCTC he and the small group working for and with him had a different specialty than elimination. They "retrieved" people. It seemed it did not matter much where a person disappeared to or how well they surrounded themselves with protection, Decker and company would find a way to pull off the impossible and come home with 'package'.
But when the tragedy happened and Decker departed, he left behind a very upset Cox. Cox took the attitude that he could have forgiven his prize agent the failure of the last mission. He could not forgive the leaving. "No one walked away from Avery Cox. Not even Ethan Decker".
The Agency, the SCTC, and Cox were not all he left behind. He also walked abruptly away from his wife, Sydney, a pediatrician who then moved into childhood leukemia research. His reasoning for the abandonment was something he did not explain and so there was yet another person not exactly pleased with Decker when we join the story.
But both Cox and Sydney will have their chance to speak their minds.