The 24th Name is an agent with the CIA.
Well, he was. For quite a few years according to him he was an operative with the Agency and spent a considerable amount of his career in the Middle East. When we meet up with him, he is three years separated from Langley and working apparently as a consultant. He does not get into that very much except to say that in that trio of years he had allowed himself to go soft. He had 'atrophied"
"I left the travel and surveillance and late-night meetings. I left the third-world and criminal conspiracies and political games. I left the gunfights and money runs and cross border car chases." He had a new job but not a new goal and so he quickly lost the edge he had owned while putting his life in danger. He will even more quickly get that hone back when he is pulled by an old friend into a case which could easily have involved his old employers.
The appellation '24th Name' is bizarre. It has a reason which the man using it lets us know quite a way into the first recorded adventure. Before then he is unnamed. After the explanation, he is still unknown to us and the few that will know him by that 24th name of Jake will be wrong though they will not know it.
"My parents gave me a name. The CIA gave me twenty-two more." That is how the man will pass on to us the fact that there have been in his life quite a variety of identities. He lets us know that when he left the Agency, he went back to using his parent-given name but what that is remains a mystery.
How and why he will assume that 24th identity of Jake is explained in due course in the pages of the first adventure.