Aubrey Percival is an agent with the Special Intelligence Service.
It is not known if the 'Special' in that agency's name is in error, really meaning 'Secret' or if this group is actually different from the official MI6. From what is said about him, it is not the same as we are told in a blurb that "ee belongs to no known organization; his power derives directly form the Head of State, His brief - To Safeguard National Security".
It is also not known what is meant by the rank of 'Captain' assigned to him. It is not a very high rank for a man of his advanced age if it came from the Army or Air Force but would be more appropriate if he had earned it in the Royal Navy.
Concerning his age, Percival is very much nearing the end of his career, being likely already moved into the low 60's. By his attitude and his bearing, those years were spent doing exactly what we watch him doing - serving in a hush-hush occupation with its requirement of keeping his thoughts to himself and telling those outside the 'business' only as much as he has to to get the cooperation he needs.
To those with whom he comes into contact, he is the epitome of the English upper class, complete with bowler hat in a time when such things were almost unheard of anymore. His manner of dress is impeccable with only the finest of material. There would be no doubt to anyone that Percival belonged to only the best of the clubs. He speech had all the expected diction and clarity that proper schooling would instill.
As another blurb lets us know, Percival "lives in a luxuriously appointed barge moored in the shadow of Westminster" and "is no more emotionally affected when frying mushrooms as a reward for services rendered by his two assistants than when he is told that the arms manufacturer he has caught selling top-secret missile systems to Russians will find his life dramatically shortened as a result."
His way of moving and talking is always with the least amount of emotion or expression; he is at all times calm and in complete control. Having said that, those who go up against him invariably find him "urbane, charming, and completely ruthless".