K-4 is a "flying spy and soldier of fortune".
The time period for the three recorded adventures of K-4 that are available is the fairly early days of the Second World War. Britain and France have both declared war on Germany but the actual hostilities have only just started though both sides have begun arms buildup. While the U.S. has declared its neutrality in the troubles, many Americans have joined their Canadian cousins and crossed the Atlantic to offer their services.
At first there is some shaky indications that K-4 might one such American. In the second adventure, a German soldier thinks he is British. It is not until the third and final mission that we are told straight out that he is indeed British. Since K-4 has chosen to not reveal his actual name to us, being uncertain to his nationality seems a minor point. K-4 is what everyone, even his closest mates, call him (which is pretty unusual, to say the least).
We learn right away that this highly skilled and experienced man is given the rank of Captain in the Royal Air Force. We are given a bit of history about him when we are told he was the "youngest flying lietutenant in the First World War with 11 victories to his credit at the close - triple that with the loyalists in the Spanish Rebellion, and dozens more in the China-Japan conflict!"
This gives us a clue as to his age. Though the depictions of him show a still young-ish individual, he is decidedly not a spring chicken. To have been old enough to have flown so many mission during the Great War, he has to have been born near the end of the 19th Century which would make him in his early 40s during these new escapades.
As the title of this entry shows, it is not just K-4 that is of interest. He is part of a trio of adventurers who have either dubbed themselves or were so named by others as the 'Sky Devils' based on their abilities in the air and the troubles they bring to the enemy. The other two members are:
Lt. Ronald Wolverstone-Clodd, "scion of a wealthy English family who gave up wealth to become an actor. With a complete knowledge of make-up, he is an invaluable aide to K-4".
Lt. Rene D'Alivergne, "French World War Ace and expert swordsman, who now flies as K-4's devoted assistant".
K-4 earns his identification as a freelance spy because of his willingness to go behind enemy lines dressed as the opposition to carry out dangerous missions, any of which would warrant his death if caught.