Nick Carton is an agent with British Intelligence.
Well, we think so. We really do not know and are not given a whole lot to go on except that in publications meant for young readers (or those young at heart) in the British comicbooks of the 40s and 50s, it was common to refer to 'secret agents' as 'special agents', so ... Then again it was also not unheard of to take a law enforcement detective and give him wide range in authority and label him the same. Again, so ... Is Carton a secret agent or the British equivalent of a federal agent? Considering the kinds of cases we see him on in the half dozen recorded adventures we have, the answer to that either-or question is - yes! Of course, since we are shown that he has his own office with his name on the door, he could be a private investigator (an estate agent as they were called then) who occasionally gets called in to help the Intelligence community. One miscreant who runs afoul of Carton refers to him as "the famous 'tec" which throws weight towards one side but 'police detective' or 'private detective' is still not known. And Carton certainly does not give us any clues.
Carton is a man of considerable determination and regiment. In the first adventure I was shown that he is in, he is grim-faced and set in his mind with a 'do not get in my way' mien about him, dressed in a suit and tie, complete with vest and fedora, as he is heading out his office - heading off on vacation! That same attitude is shown in every case; the man smiles only when he has stymied some bad-guys bad-plans.
We are never given very much information on Carton. By the looks of him, he is likely in his mid-40s, a guess aided by the fact that just about everyone, civilians and bad guys, know him by sight. He has a slopping forehead with a balding, receding hairline. He has a sharp pointed hook nose and an even more pointy chin (think Dick Tracy-ish).