Ben Treven is an agent for the Department of Defense.
Having joined the military fresh out of high school, eschewing college, he had the intention of becoming a Seal and his tenacity and his superior athletic abilities got him through. Another driving force was the guilt felt over the death of his younger sister in a car crash, an accident she was in because he had not been there to drive her. That death caused him to be estranged from his family and added greatly to his already natural tendency towards solitude.
After several years with the Seals, Treven accepted a role in a highly secretive Joint Special Operations Command (JSOC) under a man named Hort. Still in the military, Treven was in the enlisted ranks with the rank of E-8, or Senior Chief Petty Officer. His new role was in the area of black ops, more often than not involving wet works. In other words, Traven was an assassin. When the military brass ruled someone needed elimination and they didn't trust the CIA to carry it out, they used their own people for the job and Treven was one of their best.
Though he was still in the service, his job gave him far more latitude and freedom to accomplish his mission than normal. Within the mission parameters, he was able to travel when and how he saw fit, perform the job at a time of his own choosing, and get out however he could. Treven was told straightforwardly that though he was a member of the Armed Forces, should he be caught, he was totally on his own. The military would deny he ever existed.
Treven is a man used to working alone and this fits well with his natural tendency to solitude. It gives him an air of aloofness that is not really felt. He never feels superior to others as his manner suggests; he just feels apart from them. This reclusiveness works well for him as an assassin. It doesn't work so well for his personal life.