John Miro is an agent with the CIA.
Though the initials "CIA" are not mentioned directly as the agency which has employed Miro for a quarter century, it is called the Firm many times amid references to the bosses back at Langley.
Miro is certainly not much to look at, and never was. A plain man who dresses plainly and cheaply, drives a regular car, and does not have any real vices or any great passions. He does his job, well, and does on his life.
Miro, at the start of the series, is nearing the end of his career. He has a reputation for getting the job done but has made few friends in the process. This is not from any antagonistic action but rather from the taciturn approach he has to his work. Listening and observing are his strong points and it is hard to do either while talking. Unfortunately for him, self-promotion is often the best way to get ahead and Miro would never even think of doing it, let alone speak up.
This isolationism is what probably gets himself set up, the main emphasis of the series. If an agent is ready to quit and has few friends to watch his back, he is the perfect choice when meat needs thrown to the lions. Miro may not see it coming as it does but he has quietly capable of survival in the field so he can survive at home, something the people who want to label him patsy should have remembered.
The full name of the character is John Miro but throughout the series, that is only mentioned once or twice. He is known as Miro to everyone, even his wife.
Note: the first book in the series, Miro, was a runner-up for the Edgar award in 1970, losing out to Dick Francis' excellent Forfeit.