David Trevellyan is an agent with British Naval Intelligence.
Though his department is now known as the Legation Resource Unit, it is still thought of by most as its original name, Royal Naval Intelligence, having been a victim of bureaucratic political correctness. Within this organization, Trevellyan holds the rank of Lieutenant Commander. He has worked for this organization for the past 14 years as of the start of the series, making him in his mid 30's. The various missions, and there have been many, that he has had have taken him to all the continents of the world except Antartica.
According to the psyche evaluations taken on him when he first started, he is a determined independant thinking who dislikes immensely being told what to do and ever more so how to do it. Apparently, this attitude has only deepened over the decade and a half of service, making him now an agent who much prefers to work alone. Working with others, in his opinion, only slows him down.
His independent streak as a young man who's father had to move on occasion, forcing Trevellyan to deal with new surroundings, new schools, and new potential enemies. One incident detailed in the first book talked about how, on his first day in a new school, he was accosted by three olders boys who were making fun of his northern accent. When words turned to physical combat, Trevellyan struck first and hardest. Since he was the new kid and the winner, that made him appear the obvious aggressor. Trevellyan then learned both that the truth isn't always obvious to others and winning is better than not.