Mike Nelson is an agent with the S.O.E.
That acronym stands for the Special Operations Executive, a British World War II organization formed in 1940 out of three previously existing espionage groups, Department EH, the SIS (aka MI6), and Department GS(R). It was the order of Winston Churchill that these often rivalous bureaus be melded into one and as a result of this directive, it was sometimes called "Churchill's Secret Army". Another whimsical though not at all inaccurate, it earned the nickname "Ministry of Ungentlemanly Warfare".
The latter definitely fits the kind of group which would employ someone of Nelson's skillset and demeanor. Nelson is more than once referred to as an extremely cold and heartless person and he proves it on a regular basis. His hatred for the Nazis is tremendous and his determination to get the job done legendary, though only through the codename of 'Eagle'. Other than that designation, he maintains his true identity well under wraps, showing all who meet him that he has no personal life outside his desire to bring down the Third Reich.
As a weapon to unleash behind enemy lines, especially in his first mission as designation 'Eagle' which is to assassinate Adolf Hitler, Nelson is relentless. He is also extremely well trained, being an expert with all manners of armaments. This includes his Indian wrist dagger, which he can apparently unconnect, turn around, and hurl with great accuracy, all while having a pistol pointed at him by a very willing shooter. But it is behind the sniper rifle that his real skill comes through.
Then there is his abilities with languages, be it French, Spanish, or, most importantly, German. He can speak it well enough to be able to impersonate German officers on several occasions, all without arousing any suspicion.
We know very little about Nelson's life before the War and it is pretty obvious that this athletic, lean man of reddish-brown hair is no one you would want against you.
The exact creator(s) of the Mike Nelson 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 Mr. Nelson.