Jeff Stone is an agent with an unnamed American Intelligence group.
The group really has no name because it is not only top secret, it is off the books. Its operations were under the direct control of the President and known to him, his Chief of Staff, and Ian Merideth, the commanding officer of the team. Stone was the head of Operations in charge of first planning and then leading his people into action. Their primary reason for existing was to fight the constant war on terror. By being as covert as they were, their methods were not under the scrutiny of Congress or the Press.
Headquarters for this team that did not officially exist was appropriately a "non-existent import-export company". Since most of the team was out of the office on one job or another or getting some deserved but seldom provided down time, the facility did not need to be large, especially as the business did no real business.
For an outfit that did not exist, and certainly had no line item on the Federal budget, the people of Stone's team lacked nothing in the way of provisions or transportation at the drop of a hat. Similarly documentation and false identities were quickly and expertly available making it likely that unofficial connections to established organizations like the DOD or CIA had to exist.
We do not get a feel for, and are certainly not specifically told, how old Stone is nor where he comes from. No mention is ever made about his history or his relationships (if he ever had time for one). It seems certain that he was once, if not still is, in the military, likely Special Forces, but that is conjecture.
What we do know is that the missions we see Stone embark on are highly dangerous and vitally important and Stone has absolutely no hesitation putting his life on the line.