Case Lee is a freelance agent.
That is so far the best description of what he does. I could say private investigator but that would not be correct as he really does not have an office or a license and that really isn't what he does anyways. Granted, he does a lot of investigating in the numerous missions he accepts but he also does a whole lot of other things that most PIs would stay far away from.
In the opening words of the first recorded adventure, we learn a very important fact about Lee that will be a factor in lots of what he does and what is done to him, namely that he "carried a price on [his] head. A million bucks". Lee knows about the bounty but he, at that time, had no certainty who was putting up such a hefty sum. As he postulated, it could be "a Yemeni sheik, Somali warlord, Malaccan pirate, Taliban mullah - all possibilities".
A couple of other important and interesting facts are learned early on.
Lee makes his home onboard his small boat, the Ace of Spades, and has done for a couple of years. Its area of normal operation is the Intercoastal Waterway of North Carolina. He is quite pleased with his home which, though it could use some touch-ups here and there, takes him regularly from Point A to Point B at a nice pace. It also has nice hiding places for his stash of "enough firepower to start a coup".
Lee earned the enmity of those people mentioned above while serving in the military as part of the elite Delta Force. That service and the enemies it created was likely the cause of the murder of his wife three years before we meet him, though that death will take place some years after Lee had left Delta and started a life as a civilian. The bounty on his head and those of his former teammates had almost certainly been the cause.
So when we meet Lee, he has left the peaceful life he once had with his wife and put out the word that his services are available. Judging by the number of assignments coming his way, he is very much in demand.