Nick Charlton is an agent with the British Secret Service.
In the very small limited amount of adventures we are privileged to follow, we do not know a great deal about this man but we can glean a bit more.
We are told straight out in the first recorded case we have that "on being demobbed from the military, Captain Nick Charlton, M.C. gets bord with city life and yearns for action. With the aid of Spud Murphy, his ex-army batman, he decides to operate as a Secret Agent." We have to assume that meant that he applied to and was accepted by some branch of the British Intelligence community since in that adventure, a woman with vital information knows enough to head to his private apartment to seek help on a matter of national security.
Charlton is shown to be a man of athletic, slim build likely in his late 20s or early 30s. In the aftermath of the Second World War and the calming of hostilities, leading of course into the Cold War, men who would normally have made a career in the military were finding themselves again as civilians as the British government chose to greatly downsize the armed forces. Even someone who had won the prestigious Military Cross, at the time the second highest medal for 'gallantry' on the battlefield, is subject to a 'thanks for the service and goodbye'. By the way, we are not told exactly which branch of the military Charlton served but it was likely the Army as they and the Royal Air Force were most likely to have still made use of 'batman' for officers (the Royal Navy would have called them stewards).
Soon after, this man of action who is suddenly devoid of such a challenge discovered he was bored with nothing to do. Since no mention of a day job is given and we learn that he is still employing his former batman, Spud, we can make the assumption that Charlton is reasonably wealthy from likely an inheritance (there is no mention anywhere of any title of nobility).
It being unlikely that this experienced man would have just come upon the notion of doing secret agent work out of the blue, it is possible that he had been approached as a viable candidate for Intelligence work and decided it was interesting enough to do it. Spud obviously went along with the notion. There is no mention whether anyone asked the resourceful pet of Charlton, the shepherd Rover.