For the one person on the planet who's never see the Godfather films -- SPOILERS AHEAD.
Few characters in film history have displayed the cunning, charm and utter moral ambiguity as that of Tom Hagen, the Corleone family lawyer in Francis Coppola's first two Godfather films. In Mario Puzo's novel, as well as the film adaptation, it's revealed that Hagen (played by Robert Duvall) was found living on the street as an 11 year-old by pre-teen Sonny Corleone (played in the film as an adult by James Caan) and unofficially adopted by Don Vito Corleone (Marlon Brando) as one of their own. Puzo's novel reveals that Don Vito never formally adopted Tom, as he felt it would have been disrespectful to the boy's real family, who were torn apart by their father's alcoholism.
Throughout both films, Hagen remains the voice of reason and rational thinking, particularly when the Corleone family's blood is up during a conflict. In The Godfather, Hagen is kidnapped by drug kingpin Sollozzo (Al Lettieri) after Don Vito has been shot by the former's assassins. Sollozzo calmly tells Hagen to be reasonable and convince Sonny, who will become head of the family after his father's death and is notorious for his ferocious temper, to take the deal to distribute Sollozzo's heroin in the Corleones' valuable New York territory, which had been turned down by Don Vito as being "a little dangerous." When word is passed that Don Vito has survived the attack, Sollozzo is furious, but doesn't take it out on Tom, as he knows that Hagen is the only person in the family willing to deal, as he comes from a place of cool, of rational thought.
Hagen most likely developed this sense of detachment from being a literal outsider in the family, still very attached to their Sicilian roots. Hagen, whose roots are German-Irish, uses this to his advantage. Fluent in Italian, and married to an Italian-American wife, Hagen floats effortlessly between different worlds, the ultimate chameleon. When dealing with hot-headed movie mogul Jack Woltz (John Marley), Hagen never lets the older man's bullying break his icy façade of smooth charm: "Let me tell you something, my Kraut-Mick friend...," Woltz spits at Hagen upon first meeting. Hagen, without missing a beat, replies: "Mr. Woltz, I'm a lawyer. I have not threatened you...Now you have my number, I'll wait for your call. By the way, I admire your pictures very much." The negotiation, for Woltz to grant Don Vito's godson, disgraced crooner Johnny Fontaine, the lead in his new, big budget war picture, seems to be going well, with Woltz taken in by Hagen's silky negotiating. Then, over dinner, Woltz blows his stack, revealing that his beef with Johnny is over a woman. Hagen can barely hide his contempt that a man of power would allow himself, and his ego, to fall prey to cuckoldry. After excoriating Hagen with an epic screaming fit, Tom calmly rises from the table, saying evenly: "Thank you for the dinner and a very pleasant evening. Have your car take me to the airport. Mr Corleone is a man who insists on hearing bad news at once." The minute Hagen leaves, Woltz's face drops, knowing he's made a fatal error, which comes to light when Woltz awakens the next morning -- to find his prize horse's severed head sharing the bed with him!
As the story of the Corleone family evolves, with the deaths of Sonny and Don Vito in part I, and Michael Corleone (Al Pacino) taking over in the film's final act, Hagen as a character becomes much colder, his heart corrupted by the evil that has surrounded him since childhood. In The Godfather Part II, Hagen is Michael's most trusted advisor as the family business has been relocated from New York to Nevada, where their focus has turned to gambling and casinos. When Nevada senator Pat Geary (G.D. Spradlin) tries to strong-arm Michael into paying him off to obtain licenses for casinos, it is Hagen who delivers the coup de grace to the senator: holed up in a remote brothel, controlled by the family's black sheep, Fredo (John Cazale), Geary sits naked on a bed, wrapped in a sheet around his waist, in a tiny room. Behind him, a young prostitute lies dead, seemingly gutted by a sure hand. Geary is despondent, punchy after obviously having been drugged. Hagen appears, his calm demeanor washing over the senator like a wave. "This girl has no family, no one," Hagen soothes. "It will be as though she never existed. All that will remain is our friendship." It's at this moment, that Tom Hagen is revealed as the most dangerous member of the Corleone family, a man who can bend others to his will with words and manipulation, as opposed to muscle.
This is never more evident than in Hagen's final act in the film, visiting turncoat former associate Frank Pentangeli (Michael V. Gazzo) where he is in protective custody during testimony before a senate committee on racketeering (based on the legendary Kefauver Committee, which investigated organized crime in the U.S. during the 1950s). The conversation turns to how, in the old days, the Corleone family was like the Roman Empire. Hagen begins the conversation:
Tom Hagen: When a plot against the Emperor failed... the plotters were always given a chance... to let their families keep their fortunes. Right?
Frank Pentangeli: Yeah, but only the rich guys, Tom. The little guys got knocked off and all their estates went to the Emperors. Unless they went home and killed themselves, then nothing happened. And the families... the families were taken care of.
Tom Hagen: That was a good break. A nice deal.
Frank Pentangeli: Yeah... They went home... and sat in a hot bath... opened up their veins... and bled to death... and sometimes they had a little party before they did it.
Pentangeli is later discovered by his FBI handlers (one of whom is played by character actor Harry Dean Stanton) dead in the bathtub, wrists slashed. Again, Hagen brings about death through finesse, cunning and manipulation, a predator indeed. Hagen's character was excised from The Godfather Part III, released in 1990, when Paramount Pictures wouldn't meet Robert Duvall's salary requirements, replacing Tom Hagen with a new attorney for the Corleones, played by George Hamilton.
If you read between the lines, Tom Hagen represented what the prototypical American hood became post-WW II: he wasn't a "street" guy, he was a guy who went to college. He wasn't dressed in leather, but in a Brooks Brothers suit. And he got his way not by standing toe-to-toe, mano-a-mano with you, a gun to your head, but by whispering the right words into your ear -- or someone else's. Tom Hagen is one the greatest, and most complex, characters in modern American film and fiction, played masterfully by the great Robert Duvall.
And in the end, all that's left is his friendship.