Secret Agent X-101 is an agent with the American Secret Service.
It is possible that by the "Secret Service" what is really meant is the American Intelligence community since at the time of the recorded adventure, the Secret Service was not officially in the spy catching business but then again, it was the "secret" service so who knows.
X-101 is the codename for Bart Benson, a man likely in his mid 30s who has a cover job for his activities. When not fighting foreign agents and saboteurs, he is the editor and publisher of (probably) large city newspaper, the Daily Record. Which city it is remains unknown. Benson is certainly the man in charge as everyone at the office calls him 'Mister' and not his first name but he also must be a frequent investigator for when he yells that he is out to get a scoop and for them to hold the presses, no one acts surprised.
In the two short and swift recorded adventures we have, we can see that Benson is blond haired and good looking. He has an athletic build and is not hesitant to put himself in harm's way. In the first mission to learn more about the bad guys he climbs the outside of a building going high enough that a slip would mean certain death. He does slip but his agility allowed him to make "a hair-raising dive down into a near-by tree".
Other skills we know that Benson has include the martial art jiu-jitsu of which he is definitely a practiced user. Benson also knows how to fly a plane shown by his leaping into one to chase a bad guy.
In both of his adventures, Benson goes up against a spy ring lead by the "High One" though in the field the one calling the shots is the lovely and very deadly Sandra (last name unknown) who throws a mean dagger and has no qualms about dispatching Benson in a blink of an eye.
Back in the office of his "day-job", Benson has the help of his lovely confidential secretary, Pat White, who appears to be devoted to her boss and in slight awe of him but has no idea he is a reporter/publisher by day and a secret operative on the side.