Steven Flagg is an agent for the CIA.
A member in very good standing in that agency, he is, more specifically, an operative working in their "E" Department, the "E" supposedly standing for Elimination. This section also has the nickname of "Ministry Of Death" (or M.O.D.). M.O.D. is the department responsible for any wet work needed by the Agency and its agents are highest on the list of those killed in action as they go where the most danger is.
This department, run by a man named Edward Charles but referred to constantly as the Old Man, is known by only three people, the Director of the CIA, the Deputy Director of Operations, and the President. It does not officially exist and it definitely does not have its offices in the then new facilities in Langley but instead remains in the Foggy Bottom section of D.C., its entrance disguised as a small flower shop. From there an operative would go to the back room, open a reinforced door with a number-lock on it, go down a long flight of stairs and then walk the quarter-mile tunnel to the basement of a tall office building where the officers truly were.
Flagg's codename in the section is David. He is one of 19 agents that make up the E Department. Flagg is thirty years old at the start of the series and has served in E Department for several already. While very good at it, he is living proof of the perils of his profession as shown by the fact that as the series begins, he has just recovered from three gun shot wounds and is coming out of his remedial training to be certified fit.
He is incredibly confident, surprising since he was shot and almost killed, and faces every mission with certainty. He is brass without being overly cocky.