Thomas Beckett is a U.S. Marine sniper.
He is not a spy though his assignments for the Marines, especially when seconded to other government agencies, some of whom do not want to be acknowledged, definitely means that he works unofficially as a covert operative. As a sniper sent into foreign lands to eliminate a particular target from a distance and then get out without leaving any trace of his presence - minus, of course, the dead body on the ground, he could hardly be expected to work any other way other than covertly.
We meet Beckett in the first recorded adventure as he begins yet another assignment after a long and highly impressive career. He holds the rank of Master Gunnery Sergeant, the highest enlisted rank possible in the Corps. This shows a couple of important pieces of intel on him. The first is that he has had to serve for likely just over two decades to achieve his high rank and as he is apparently around 40 years of age, he undoubtedly joined the Corps when he got out of high school at age 18. The second piece of data is that no one achieves that position of Non-Commissioned Officer without an impeccable, impressive service record and since his area of expertise is silent target elimination, he has to have marked up a very high score.
It is important to comment on these things because we are told so very little about the man's history outside of his being probably the best that the Marines (and therefore the entirety of the American Armed Forces) had to offer. We do not know of any family he might have had back home, except for the fact that later in his recorded exploits, we learn of a grown son who has followed in his father's footsteps in line of work if not the actual Service.
What we do know is that Beckett's ability to infiltrate virtually any terrain, find the best vantage point that is not immediately under observation, wait for the right moment however long it takes, dispatch the target (shoot him or her) and then get out again is something that makes him very attractive to groups like the CIA, making his adventures qualify him for membership here and makes his missions a lot of fun to watch. But where it really becomes interesting is when he is asked to take out someone that the CIA is doing business with. Then things get really dicey.