Steve Battle is an undercover agent.
Exactly for whom he works undercover is not known for the first two adventures but in the third it is revealed that he is the "ace trouble-shooter of the U.S. State Department's Counter-Espionage Division". He seems to be headquartered in the nation's capital where he "finds plenty of fast, dangerous action".
Battle may go with the subtitle of "Undercover Agent" but the adjective seems considerably misplaced since a whole lot of people know him on sight and ask for his help in solving some pretty nasty problems so there is little doubt they know what he does for a living. This despite the fact that without trying to wear any disguise, he is a dark-haired operative for the first three adventures and a blond for the last two.
Having said that, at least many of the people Battle is familiar with are either high-placed in the government or rich, such as Hugo Belashe, the very rich leading Balkan munitions dealer with whom he was on a first-name basis, at least while the man was alive.
Battle is definitely a go-getter who is not the list bit hesitant to jump into things as shown when he at a moment's thought dived off a naval vessel to grab a rope dangling from a launch (why was the rope there in the first place?) and body surf to keep tabs on an enemy spy.
He also has a strange attitude towards the dangers of his job, demonstrated when after taking on a spy ship's crew singlehanded, Battle dismisses his work by saying "Fighting a mob like that is easy once you've tried it."