Tom Dupree is an agent with the Bureau of Diplomatic Security.
According to that department's website: "Diplomatic Security is the federal law enforcement and security bureau of the U.S. Department of State. Tasked with securing diplomacy and protecting the integrity of U.S. travel documents, the Diplomatic Security Service has the largest global reach of any U.S. federal law enforcement agency, with offices in 29 U.S. cities and in more than 270 locations around the world."
Dupree is a highly decorated and experienced special agent with that group, having "spent twelve years overseas guarding embassy staff" followed by "another three in the office of investigations and counterintelligence". All of that had worked him up to what most would consider a "career summit", head of the Secretary of State's proective detail. As the first recorded adventure is taking place, he is winding down from that tour and is about to be transfered back to the States for a cushier, albeit more boring finishing assignment.
Having given so much of his adult life to his job, Dupree is still single. His last serious girl friend had ended things a year before with the honest explanation that she saw her dentist more than she saw him. Dupree is honest enough with himself to know she was right. Whether he would ever find a special someone is something he has put off until he finished his constant travel. While settling down is not in his top priorities, he definitely has a nice place to do so when he finally opts for it - he somehow owns a nice chunk of land just outside Arlington County, on the banks of the Potomac with a 3000 square 'yard' two-story farmhouse. Definitely something nice to come home to.