'Brass' Bancroft is an agent with the American Secret Service.
A blurb on packaging of the four adventures we have of this fellow states he is "Lt. 'Brass' Bancroft, a stalwart Secret Service agent who risked life and limb to bring to justice illegal alien smugglers, counterfeiters and enemy spies."
The use of a quote around the first name obviously indicates it is a nickname but what the real name is, I have not yet determined and it is really doesn't matter because everyone, even his girlfriend, calls him 'Brass'. The 'Lt.' comes from the rank he held in U.S. Army Air Corps.
The activities we follow Bancroft in take place in the two years preceeding America's entry into World War II. The last one shows the nation's nearing the conflict by supplying its allies with supplies but as yet the U.S. was maintaining its neutrality and as a result, people with Bancroft's skills as a pilot were not as in demand by the military as they soon would be. Bancroft is earning a living as an air jockey for a small airline while also 'spending time' with that company president's very attractive grown daughter, Pamela.
While Bancroft is not an actual agent when we first meet him, he has apparently already tried to join and is, I believe, on a waiting list. When an agent investigating a particularly nasty group of smugglers of illegal aliens show just how nasty they can be, the head of the Secret Service team approaches Bancroft with a request for him to go undercover to join the gang and fight from inside. This will require Bancroft ending the life he was enjoying, getting arrested on a trumped-up charge, and taking on the entire criminal outfit without yet receiving any formal training.
Bancroft will continue with the Secret Service as a full-fledged agent (with training, I assume) from then on but it will not be the last time that he will be forced to go undercover and face an entire gang, or fifth-column cell, on his own save for the help of his old military buddy and now steady sidekick, 'Gabby' Watters. Re 'Gabby', while that is also obviously a nickname, it is one that fits him as surely as 'Brass' fits Bancroft.