Jack Regan is an agent with MI6.
We are given a succinct, accurate, and, for Regan, ultimately demoralizing description of him and fate when in the introduction of the first recorded adventure, we are told of an important but highly dangerous mission that the folks at Vauxhall Cross devised and how what was needed "was a man they could trust, a man they could rely on, a man they could betray".
Regan's journey to his employment with the Secret Intelligence Service came through his time spent in the British military, first as a member of the prestigious Paras, aka the Parachute Regiment, and then the even more elite and impressive SAS.
Regan is good with languages. "My Korean wasn't good. My Chinese was okay, and my Russian was excellent". He had incentive to begin his foray into linguistics when as a young teenager he had a teacher named Olga with "tits to die for".
When we first meet him, Regan is married to a woman he is very much in love with, Samantha, who is apparently pregnant with their first child. She does not know of his current employment and therefore the kind of missions he is sent on; she "believes I am still with the Regiment".
The adventures of Regan, however, are not really so much about his missions for MI6 but rather about the ramifications of the last mission he is given, especially after he is betrayed and forced to live in horrific imprisonment in North Korea, a white Imperialist at the mercy of that tyrannical regime. Six years is a very long time to think about what went wrong leading to his capture and who might have been behind the probable betrayal. And why.