Sam Spolo is an agent with American Intelligence.
Exactly who he works for and what he does is best summed up in his own words which come at the beginning of the first published adventure:
"My name is Sam Spolo and officially I do not exist nor do I have a title. Unofficially, I am known as the Spy Smasher. Any place in the world where our interests are threatened by the presence of an enemy spy ring, that's where you will find me. I may be called upon by any agency of the United States or, in turn, I may call upon them for help. I am almost a lone wolf and I have but one assistant, Ben Riley. When needed, we have unlimited funds and unlimited power."
Words of a similar nature begin each of the adventures of Sam Spolo. This description will change a bit here or there as time goes by. The people who might request the services of Spolo will get extended towards the end of his recorded career to include nations friendly to the United States. His mission remains the same, though, and when he talks about "any place in the world" he most definitely means it. Europe is his most common destination but he truly goes all over the place.
His sole assistant will change a third of the way through the series to become Ben Manners [I do not have he story and reason for the change]. Whether it is Riley or Manners, the role of assistant is by no means that of a flunky meant only to admire and praise his boss - each Ben will useful information or suggestions to aid Spolo in coming up with a solution. It is a rather good relationship.
We learn very little about Spolo in his 30 adventures - certainly nothing of his past. We can see that he is very intelligent and highly observant. He can and does discern clues that are so part of the background that most miss them. He is logical enough to take a couple of facts and reach a correct conclusion, hence his tremendous track record of successes.
We also see that he is by no means an automaton. He can be surprised. He can joke and enjoy himself. He is not above the occasional vacation or R&R trip to unwind. He has an appreciative eye for a pretty lady.
Mostly, though, we know he enjoys his job and he is very good at it.
Note: Apparently the covername of Spy Smasher is a popular one. In 1940, Alan Armstrong donned a cape and goggles and started smashing spies under that moniker. 60+ years later his granddaughter would not dress so oddly but would also take the covername as she went about smashing spies. In between, Sam Spolo, of no obvious relation, called himself that as he flew around the world smashing spy rings.
Note: The creator of Sam Spolo remains unverified but evidence points to it being Joe Gill, a very prolific comic book and short story writer who worked for Charlton Comics during this period and was known to have written text stories for that company's Fightin' military line. As a result I have credited him as such. If I am wrong, let me know.