Michael Brennan is an agent with the CIA.
Officially, he isn't. If any could look at the Agency's books, which they cannot, there would not any record of Brennan being an officer with them. Brennan has the uneviable fun of being a career NOC, or non-official cover, which is a nice way of saying that if things go south, as they often do, he is on his own.
Unofficially, though, he is very much an operative with the CIA and has been for a good number of years, enough to get a decent reputation as a man who gets the job done. The type of job might change from mission to mission but the style seemed to remain constant. An NOC got called in when nasty has hit the fan and the chance of cleaning it up slim. Luckily pulling off the impossible is something Brennan has gotten good at.
Brennan came to the Agency in a traditional way - through the military. He got his degree from The Citadel in South Carolina and from there he went into the Service and got noticed, bringing him eventually to the CIA. Once it, he has become a valued asset and the man to be called upon.
When not on assignment, Brennan enjoys keeping in shape by taking part in triathlons, a very important part of his survival regimen as he has learned the hard way that in the field, especially undercover, training and keeping in the kind of shape he must to stay alive is often either impossible or outside his adopted identity. So getting back into condition when at home is vital.
Other than his devotion to physical training and a very friendly relationship with a lady used to his disappearing for long periods of time, the most telling thing we know about Brennan is how little we know. Brennan is a very private individual.