The Godfather is a great film. So is The Godfather Part II. And part of what makes them great are the great characters.
Tom Hagen is an Irish-German adopted son in the Corleone family. As a non-Italian, he's excluded from certain mafia-related events. But his career as an attorney makes him an invaluable asset to a notorious criminal gang.
When his credentials are questioned by another attorney in the film, he calmly explains, "I have a special practice. I handle one client." That client, of course, is Don Corleone.
Tom is a quiet character, but his presence is a valuable contrast to the hot-tempered emotions or conniving schemes of the Corleones around him. And perhaps (or so I like to think) it's because he has a slight dispassion in his role as advisor and counselor. He's allegedly based, at least in part, on University of Southern California Law School alumnus Frank DeSimone. And it's for Tom's profession as an attorney that he's the Fictional Attorney of Month.