Nick Douglas is an agent with the British Military Intelligence.
That statement is based on supposition, not hard fact. In fact, other than inference from the covers of the few recorded adventures there are, I know nothing about him.
The second adventure we have of his says in its caption that "He walked alone - and fought alone!" That would indicate he is a loner. [Note on that cover. It shows an identical blond man in a suit kneeing the jaw of an identical Nazi officer as can be seen in the first issue of .]
The third adventure shows only that he is an operative with the phrase "Non-stop action with Nick - Secret Agent!". The fourth tells that "Nick Douglas hurtles into action!" while showing him falling to the ground (but he is getting a shot off). The fifth indicated that he is in combat with jungle rebels and reaffirms hims being a secret agent. The sixth show him still in the Far East on another vital mission.
From his depiction, we can assume he is in his early 30's and has wavy blond hair and a muscular, athletic build.
The action that fills Douglas' time takes place during the Second World War. In the first adventure we have of Douglas, he is recalled from vacation for a mission of "vital importance". His easy going mannerism shows he is ready and willing for danger at any time and that he has a sense of humor. Since the mission was to head into Germany to impersonate someone, it must be assumed that he spoke German like a native. And since he would be standing so close to high ranking officials, including Hitler, who knew the real man, Douglas has to be a master of disguise.