Mills is an agent for MI-6.
If you wanted someone appearing less like a spy, Mills might be your man. He is described as in his late forties with hair that has all gone to white. He is medium height and rather stocky and his roundish face is already well on its way to heavy jowls. His eyes tend to water easily; making him look like he is about to cry and his voice is soft and often hesitant.
In addition to the normal work that comes his way, Mills spends much of his professional life tracking Nazi war criminals, one of which has become almost an obsession with him two and a half decades later. In that endeavor he works on the side with a fellow hunter named Moshe Singer and has been successful in tracking down several moderate-level Nazis over the years.
Mills is single and figures he will always stay that way. He likes women and will find one on occasion to spend time and his bed with but for lasting relationships, he knows his work and his personality would never let it work. Twice before the series and once during it, he has been tempted but each time something happens to pull him away so he remains alone and often lonely.
Of the three books in the series, Mills is the main character in the first and a very important supporting character in the third, especially towards the end. The middle book is connected to Mills only in that he helped train some of the people in it and there are numerous references to him and his "retirement".
While he certainly has a first name, no one ever uses it if they ever knew it in the first place. Mills is just called Mills by everyone.