Nick 'Bullet' Carter is a soldier in the British Army.
During all the time we follow Carter in the many adventures the man has during World War II, we learn one important fact - the man loves to ride motorcycles! He steals one in the first recorded tale and becomes semi-attached to it, loathing to give it up and using it to accomplish his many varied missions.
When we first are introduced to him, he is in a German P.O.W. camp and from the sound of things, he has been there for a while but not content. In fact, he has just been recaptured. According to the dialogue between Carter and the Kommandant, this is not even close to the first time he had escaped. He has been dragged back to the Stalag and has just finished getting his "you can never escape" spiel from the irate warden when he espies a German cycle left unattended. He breaks free and manages to find a soft spot in the barricades in which to make his brazen jailbreak.
Getting out of camp was tough but getting away from the many guards sent after him would be even harder but the key thing we learn from this event and the thing that will pay a recurring role in all the adventures he will have subsequently is that Carter can ride a motorcycle like it is an extension of him. Whether it is turning on a dime, leaping over walls and people, or plowing into a cache of armed men, he wields the bike like the professional he is.
One of the obstacles he meets while making his getaway turns out to be a fellow soldier trying to get back to his side of the war having done a job for British Intelligence and needing to get his vital intel back to base. Carter has him hop onboard and together they get free and it is then that the spymaster at HQ offers Carter the job of going back into the fray using his riding skills to get places fairly quickly and often unobserved. Carter, loving excitement and riding, jumps at the offer.
The "name" of this series is 'They Can't Stop Bullet'. The bullet they refer to is Carter, having gotten that nickname when he won the Isle of Man motorcycle race before the war started. He would prove the fact that he was virtually unstoppable many times thereafter.
The exact creator(s) of the They Can't Stop Bullet! series has not come to light yet. The industrious fellow spy-fan who has been providing all the information on this series for me has learned that the magazine was the creation of five people: Pat Mills, John Wagner, Gerry Finley-Day, Dave Hunt, Doug Church.
It is probably that one or more of these creative souls came up with the idea of Nick 'Bullet' Carter.