Grant Stevens is an agent for the President.
When the series begins, he is the best investigator the U.S. Naval Investigative Service (NIS), the forerunner to the modern NCIS. The adventures take place likely in the early 80s (possibly very late 70s though this is not expressly stated but can be inferred from events and time-lines) and the NIS was the main department in the Navy for handling all major criminal activities including espionage. It is with the latter that Stevens excels.
Stevens is, again at the start, a Commander in the U.S. Navy and more specifically and importantly, he is a former SEAL. In fact, he is one of the very first SEALS to have existed as he got his start with that Special Forces group in 1962, the year the SEAL program came into existence. He was a member of the first major mission the SEALs took part in - a foray into the jungles of Cuba to find more proof of Soviet missiles on that Caribbean island. He was a very junior Ensign at the time, likely just having graduated from the Academy (supposition).
As a result of the actions in the first book, Stevens receives a promotion to Captain and it is stated that he is at the time the youngest man with that rank, likely making Stevens at or near 40 years old, within the reasonable range for the hijinks he gets involved with but still making his physical actions even more impressive. Years of action and a dedication to staying fit have served him well.
After nearly a half dozen recorded adventure, Stevens opts to leave the Service and it is at this point that his working on behalf of the President starts up as a quartet of wealthy patriots approach him with the plan to establish Team Alpha Tango, a small group of highly trained operatives working outside the normal chain of command to take on the tasks deemed vital for national security by the President.
Stevens is a native of California having been born and raised in a small community north of the Bay area near the coast, giving him ample time to take part in one of his favorite sports, surfing. Standing 6'1" he is in excellent shape. He has brown hair and eyes and is fluent in Russian and Japanese with a spattering of other languages along the way.