Butler is an agent for the private Bancroft Institute.
For many years, Butler, who has a first name but hates it and therefore doesn't use it ever so the reader never finds out what it is, worked as an agent for the CIA. On the one hand he was considered, certainly by himself, to be one of the top men the agency had. On the other, he was extremely opinionated and vocal about his dislike of the agency's policies. This led to his having every supervisor he was given.
At the start of the series, the vociferousness of his objections has led the agency to finally fire him. Suddenly without a job and feeling understandably sorry for himself, he is open to a meeting with people from the Bancroft Institute, especially when it appears that the Institute has set him up to be arrested for murder unless he helps them.
The Bancroft Institute is comprised of dedicated men and women determined to thwart the evil machinations of the vast military-industrial complex of the western world. Dubbed Hydra by the Institute, this loose federation was itself dedicated to the subjugation of the free world. Only through the Institute could their actions be countered. They disguise their real work by posing as an international think tank and research organization.
Butler is two things in this series. He is a tireless agent on behalf of everything that is decent and good. He is also one of the world's greatest lechers who has the attitude that if there is a free moment, it should be spent having sex. Of the two, the books seem to spend more time on the latter than the former but that may only seem so because it drags so badly.
The one redeeming point to the series, if it can be called that, is that the agents of Hydra are even nastier and more unpleasant to be around than Butler, definitely a case of the lesser of two evils. While Hydra will replace its leadership if that head is cut off, at one point its ultimate leader was known as the Swami.