Jim Stagg is an agent with British Intelligence.
His period of activity was during World War II with some events taking place in 1940 and others in 1941. His area of operations was behind the German lines though that did vary with the mission, be in in France, Holland, Italy, Poland, Yugoslavia, Greece, Russia, or deep inside Germany itself.
His age is likely, by the looks of the man, in his late 20's or early 30's. He is moderately good looking and in very good physical condition to pull off some of the things he does but overall he is average in appearance which is extremely valuable to him as he takes on various personas like a poor farmer or a tradesman or even a German officer. Languages have to be one of his greatest skills for he can talk like a native in numerous European tongues, giving him expert cover when he goes into one of his disguises.
He has a nemesis in a Gestapo officer named Major Kroger, or perhaps it is more accurate to say that he is the thorn in Kroger's side. Nearly all of the ten adventures we have of his show him going up against Kroger or having Kroger show up at the end surmise who the [expletive] Englishman likely was. This is impressive because it is said that Kroger is Gestapo's top interrogator so if Stagg had him as a personal adversary, Stagg must have been doing a lot of harm to the Germans.
We know nothing about Stagg beyond the fact that he is hailed as Britain's most daring secret agent. Certainly by the looks of his actions, he is one of the most intrepid and considering all the places he pops up, one of its best traveled. And he has friends or acquaintances in many different areas. At one point he bragged, "I have contacts everywhere ... even in Gestapo Headquarters. I have to know everything I can, just to stay alive!"