Luke Carlton is an agent with MI6.
More technically, he is, when we first meet him, an "ex-Special Boat Service commando, now under short-term contract to the SIS". That is how he is described in the first recorded adventure but that 'short term' gets changed over time such that in the third adventure, he is described as "a fully fledged case officer in the Secret Intelligence Service".
Carlton is a veteran of "twelve years in the Royal Marines, the last four spent with Special Forces in the SBS". When he opted to leave the service and accept an offer from the clandestine service, he was placed under the control of a handler named Angela Scott, a short, petite "woman of few words". The two work well together; she reluctantly lives with his tendency to do things his way and he admits to himself that "he could do a lot worse than" her.
We are told that he is one of the few field operatives who has extensive experience with close quarter combat and the use of firearms so it stands to reason that when a mission comes up that is likely to result in needing those skills, he is the man to call. Though he might indicate otherwise to his live-in girlfriend, getting those calls is something he looks forward to.
From the viewpoint of that girlfriend, Elise, with whom he has been for nearly a year, "He had the loose, easy movements of someone capable of immense speed and power. His face was angular, lightly tanned, beneath short, sandy hair. Grey eyes. Was he a model? God, she hoped not - she wasn't going to make that mistake again. No, the broken nose and weathered look told her otherwise. This was not a man who spent a lot of time in front of the mirror. There was something paradoxical about him: a blend of danger, adventure ... and security. She felt safe in his presence."
The first adventure we follow Carlton on takes him to Columbia, a country he had lived in as a child with his parents up until their deaths in a car accident. His understanding of that land and his native control of the language help him immensely there. Not quite so with the next couple of missions which will have him a hot, largely arid Iran and a cold, frozen Arctic.