Duncan Hunter is an agent with the CIA.
Okay, maybe not so much but he does work for them and in his own way he is an operative who gets involved in some pretty dangerous missions so in that regards my statement is correct. If you asked him, though, he would say he was a Marine first and foremost. Then a Marine pilot. Then a contract pilot for the CIA. Agent is something he would not say.
"Duncan Hunter, a retired Marine Corps fighter pilot, flies a top-secret airplane with his mentor, Greg Lynche, the sometimes uber-liberal retired CIA Chief of Air Branch. Together, in their quiet airplane, they execute some of the CIA's most sensitive airborne counterterrorism missions under a Special Access Program."
That plane is one of the main characters of the adventures, flown as it is by Hunter, someone very talented in the cockpit. It is a highly modified YO-3. According to Wikipedia, "the Lockheed YO-3 'Quiet Star' was an American single-engined, propeller-driven aircraft that was developed for battlefield observation during the Vietnam War. It was designed to be as quiet as possible, and was intended to observe troop movements in near-silence during the hours of darkness." The descriptions given for the further modifications made by the CIA technicians indicate that as quiet as the original design was, the new version is considerably more advanced therefore the plane is astoundingly silent.
So the impressive working, though hardly impressive looking 30-year old aircraft helps tremendously get Hunter into and out of trouble spots. Once there, of course, his continued survival is all up to him and his other talents. Luckily those are quite a few as well, starting with the fact that he was for many years a Marine and as such is still very much a Marine.