Jamie de Grispere is an agent for the British Crown.
He does not go looking for that position, though considering the fact that there were no official organizations for such activity back in the time period these adventures take place - the early 1400's - that is understandable. What James, his real first name, is anticipating as his calling is to become a knight in the service of his king. That will happen but along the way he will also become a very valuable servant of the Crown in less chivalrous endeavours.
Though his name shows him to have French origins, de Grispere is very much British. He is still fairly young (not sure of his exact age but I would peg it at around 20) and is a page to a powerful knight and landowner in the northern regions of England. He was afforded that impressive position thanks to his wealthy merchant father who is a lifelong friend of that man.
It is not just nepotism, though, that allowed de Grispere to be noticed and appreciated. Early on, he shows himself to be highly intelligent, impressively athletic, and quite deadly with sword and bow. It will be that first quality which will cause him to be recalled back to London and placed into the service of King Henry IV's feared spymaster, Sir Richard Whittington. It will be the latter two skills, however, that will keep him alive on several occasions as he goes about the missions that Whittington will assign him.
The title to each of the adventures of de Grispere refers to "A Knight and a Spy" and through the actions as the latter, young Jamie quickly becomes the former by the end of the first recorded adventure. He does so through merit and skill but he would be the first to admit that he survived also because of a new friend and loyal assistant, an Italian by the name of Cristoforo Corio who saved the life of de Grispere's father and who would quickly become pals de Grispere. The two men are considerably different in history and attitude but each really complements the other and both would likely have died several times over if not for the other's intervention.