Jason Drake is an agent with the CIA.
More specifically, he is an assassin who removes from the list of the living those that the Agency has decided no longer deserve to be breathing or who pose a threat to the United States so as to warrant such a termination.
When we first encounter him he is stumbling down a side street off Pennsylvania Avenue wounded from an attack and struggling to stay upright. As he haphazardly passes a group of Washington elites leaving an art exhibit, we are given a good example of Drake's opinion of them: "To them, Drake was just a blemish, an inconvenient truth, nothing to be concerned with - he was the help. Drake had more in common with the janitors and maintenance men who mowed their lawns and cleaned their toilets than with them." In response, "he couldn't give a rat's ass about the swamp creatures that inhabited the capital". That sums up Drake's attitude towards a lot of things.
A bit later in a flashback a few months before the situation above, we learn that Drake had been for several years part of a now defunct black ops division of the CIA called Terminus. As Kate Price, the former head of the group, tells about it and Drake, "We were the ones you called when you had no other option. But things went wrong when one of my officers went rogue. He [Drake] killed a U.S. senator, and disappeared. The CIA have been looking for him ever since. Due to the nature of the incident, Terminus was disbanded."
"Each one of my Terminus officers had been given psychological enhancement training, injections of various drugs to increase their endurance and mental faculties. We were told they were safe... maybe they weren't."
Since the time of his vanishing, Drake has been living in San Juan where "he keeps to himself, stays in the shadows" and "has earned a Robin Hood-like reputation down there". At least until Price, being faced with a situation that only Drake can solve, offers him a chance at clearing his name.