Gregg Kaplan is an agent with the CIA.
When we first meet this highly capable and downright deadly operative, he is in an extremely stressful occupation but hardly one that would require the ability to put a bullet into any part of the anatomy he wanted or to defuse a ticking bomb. Well, the latter might be close because the stress level would be similar. Kaplan is an air traffic controller and has been one for quite a few years.
Prior to that, we learn in the first recorded adventure, he had been in the U.S. Army Special Forces. He had served as a Delta operative for a few years and has one of its tattoos to show for it. What his missions in that organization were is kept in his head for that part of his life was classified.
After a while, though, he decided that a calmer life was called for, though he obviously could have chosen something less demanding. Or could he? After a fair amount of time serving in one of the most elite outfits in the world, switching to a job that required constant attention and focus sort of makes sense.
As we learn in that first book, trouble will force its way back into his life and when that problem is finished, so is his life as an ATC. The same crisis that ended one career opened the door for another and Kaplan moved to the next stage of his life - an operative with the CIA. The same hassle that changed Kaplan's life also introduced him to a man who would become a friend and partner (for a while time on the latter), but just as Kaplan had his own life before he met Pendleton, he would have his own after they part company.