John Chalmers, aka Blitz Buster, is an agent with the U.S. Secret Service.
I do not think that the Secret Service to which he belongs is the same one we know formerly part of the Treasury Department, though. That bureau has civilian agents trained in law enforcement and protection. The Secret Service in this two-issue comic, is military and working to uncover potential enemy secrets.
"Potential" is the operative word here for the first mission depicted takes place in September of 1941, four months before the surprise attack on Pearl Harbor that brought the Americans into WWII. Though the U.S. was not yet actively involved in conflict with the Germans, the mission for Chalmers is to infiltrate the German command operating in southern Europe to learn its secrets and methods. This was to be better prepared for when that blitzkrieg ability might be turned on American forces.
Chalmers is a major in this Secret Service and by the friendly, joking way that he is addressed by his commander, he has been around for some time and achieved a fairly good success record. The Major is in his mid-30s by his appearance and is a native German speaker. He is also, according to his boss, a master of disguise which makes him able to impersonate a dead German spy.
His ability at subterfuge are not needed for his second, and last, escapade. For that he needs fortitude and tenacity to weather a couple of pretty shaky mishaps and still take on an entire German U-Boat. He has the require spunk, of course, and is able to show those pesky Nazis a thing or two about American grit. And to show the lovely stewardess he keeps saving.