Bert Mangum is an agent with the AIA.
That is the Association Intelligence Agency. It does not exist. Just ask the Association of Planets (AoP), the governmental federation a whole lot of planets belong to, The AIA does not exist because its existence is "a myth. There's no such outfit. The AoP says there isn't anyway". Looking into any official documents and you will find no proof whatsoever of the AIA. "There were stories. There were conspiracy theories. But they were all nonsense. There was no such thing as the AIA."
"Meanwhile, at the AIA, they were blithely unaware they didn't exist." If you head to Ashur, the capital city of the planet Mardouk, you can find the AoP's Agency for Interstellar Trade which most who work there call 'the Agency'. You can go it, if you want, take the tour of the very boring museum the facility has and visit the gift shop. You just cannot go above, or below, the ground floor. But then, why would you want to?
By the way, the AoP is not the only collection of star systems governments. There is the Gaston Alliance for one and the Star Nation of Abelon for another and apparently others within reach, not to mention a good number of independent planets. There are a lot of stars in the galaxy and most of them have planets.
Mangum has been an operative with the AIA for a good number of years by the time we meet him and he has completed successfully an impressive number of missions to a large number of planets both inside the AoP and outside. This tall, slender, very good looking man is highly competent and not at all nervous about putting his life on the line because he knows he is pretty darn good at his job and is likely to come out a scrape in one piece. The same is not likely to be said about those he fights.
Mangum has a partner in his efforts though this bosses do not know about it. No one knows, really, except Mangum; not only that he has a sidekick of sorts or that such a creature as Sam exists at all. Sam is an alien with some pretty unusual characteristics. And by alien I am not talking about a foreigner, documented or otherwise. Sam is another whole species of life from a distant planet: "the only intelligent alien life humans had yet discovered. And they were only discovered by Mangum when he was shipwrecked - marooned, actually - on Sam's planet. But the aliens didn't want their existence generally known by humans, and Mangum had agreed to keep their secret, even from his employer."
"The creatures didn't know what to make of the human, but one of them had decided to tag along when Mangum was finally picked up. He said it sounded interesting." That one was named by Mangum (or himself, who knows) as 'Sam' and "Since that time - years ago, now - Sam had traveled with him, sometimes helping out, but mostly observing humans and acting as a sounding board for Mangum."
I would describe Sam, the oh-so-definitely-not-human, but .. no, you should read it yourself.
And I should mention that while Sam the alien is not known to anyone other than Mangum, Sam the golden doodle pet of Mangum who goes wherever Sam goes, is. Morphing into different shapes is one of Sam's amorphous abilities and when he chooses to look like a dog, people do not see (and ask questions about) the antennae with the eyes at the end atop his probable head. Picking that disguise was interesting, though, because a golden doodle had "All the intelligence of a poodle with the temperament of a golden retriever. Which was good, because poodles had bad attitudes and golden retrievers were dumb as a bag of hammers".