Peter Marlow is an agent for the British Secret Service.
Marlow is a good, honest agent for British Intelligence who has the misfortune of having a wife who has left him for another agent. Furthermore, he is working for a double agent, every bit as devious as Philby or Blount. While trying to save the former, he becomes a pawn and then patsy for the latter and his entire life is changed dramatically.
Marlow is not a flashy secret agent speeding along in a sports car to Monte Carlo. He is a very earnest man with a methodical approach to things who has no asperations to be more than he is, a middle class man with a wife and a steady job and happy to maintain the status quo.
He certainly did not start out to be an espionage agent, or a patsy. He had been a teacher in an English school in Egypt when he and at least one other teacher were approached with the chance to report on events back to London. After a time, the offer expanded and eventually he left teaching to become a full time agent, using his expertise in the Middle East and a good knowledge of Arabic as his basis. A good deal of this is revealed in a long flashback in the first book, an important read to understand all the rest that transpires.
As the series progresses, his status goes from okay to horrid to strange to apparently good but in keeping with the whole concept of deception, you never completely know for sure. Marlow remains a man who keeps trying to be good and normal and happy but it seems the Fates do not agree with that concept.