Tom Locke is a mercenary.
Some might elevate his career to being a security consultant but it is unlikely Locke would do so. He is comfortable with what he does and does not play games with it.
When we first meet him, he works for what seems to be a major player in this line of work, Apollo Outcomes. He has, according to his own words, "been the Apollo man in Africa for more than a decade: raising small armies for U.S. interests; preventing a genocide in Burundi with twelve competent soldiers; defeating a warlord in Liberia without firing a shot; 'shaping the environment' in Sudan to make way for American foreign policy. Standard stuff." The mission he is completing at the beginning of the first recorded adventure was different for him, however, with the objective being "to seize, protect, and operate major oil fields, on foreign soil, for an American oil company, in the middle of a civil war."
Locke does not fit into the stereotype of a mercenary, though. For one thing, he is not uncomfortable or ill at ease in a suit. He actually owns several of them and they fit well. For another, he can deal with boardroom meetings as easily as he can dark alley rendezvouses or campfire powwows; he knows that there is a potential for bloodshed at any time so tread lightly, don't lose your cool, and always watch your back. And then there is the love of opera.
His background does go along with any "standard" merc career path, if a bit more prestigious. He had served for several years in the U.S. Special Forces but chosen to get out when his enlistment ended and gone to college. He had been a graduate student when first approached by a recruiter who implied he worked for the State Department but really was with the CIA. He had not decided yet when the planes hit the Towers. Some time later he switched to the private sector where his work was the same, just the name on the pay stub was different.
Some mercenaries do the work because they have nothing else they can do. Locke is different in that he loves the the excitement and needs the challenge. He is smart and experience enough to know that "sometimes 'best man for the job' just meant the least informed".