Jon Reznick is an agent with the CIA.
Is - sort of. Possibly unofficially because the Agency does not immediately answer affirmatively to that question when asked by the FBI or perhaps they just like keeping up the old rivalry. On his tax returns he lists himself as a management consultant, which lends credence to the unofficial aspect.
Then his employment switches to the FBI though this is even more tenuous as it is on a "consultancy basis" presented by Assistant Director Martha Meyerstein, "in charge of the Criminal, Cyber, Response and Services Branch". This woman plays a major role in the life of Reznick from the time we meet him. She first is someone leading the charge to apprehend him then she uses him in the consultant role and then is there to back his play on a thing or two and once in a while gets gets a bit irked at his antics.
Prior to both of those, Reznick had been for many years a member of the elite Delta Force. He was a legend in the outfit, according to some. "The guy's a ghost. Black ops. No one knows or is admitting whose responsibility he is but ... we believe he's carried out countless assassinations on behalf of the American government for the best part of fifteen years. Former Delta Force. The unit is also known as CAG, short for Combat Applications Group, for those familiar with Fort Bragg. This guy, Reznick, is something else. Been there, done that, got the T-shirt. Also got a major beef with authority, according to his file."
During that time he met and courted and married a very sharp lady in New York, a partner in a prestigious law firm. They gave birth to a daughter named Lauren. Life was good for Reznick. Then the planes hit the World Trade Center where his wife's firm had their offices and his life darkened when hers ended.
"He was drinking way too much and was considering leaving Delta. But suddenly, a shadowy new world opened up to him. A world of false passports, false identities, fake IDs and synthetic suicides. A world of surveillance and shootings. The more he did the less he felt. He got a call, he did the job. No questions asked. And the money was great."
Life will change again during the first recorded adventure we have with him, and the new opportunity of working with Meyerstein and the FBI will come around but Reznick's "beef" with authority and his go-his-way will still shine through.