The Godfather
In the film, Michael Corleone calls Hagen his brother. Hagen is introduced as an important member of the Corleone family. As a child he grew up in a broken family, the son of an abusive alcoholic. Hagen is saved by Sonny Corleone, who finds him on the street and takes him in to live with the Corleone family. Vito Corleone becomes a surrogate father to Hagen, but does not officially adopt him out of respect for the boy's father.
In the novel, Hagen offers to work for Vito after going through law school, knowing full well that his adoptive father is the most powerful Mafia chief in the nation. Vito is more than willing to take Hagen into his employ, telling him that lawyers can steal more than a phalanx of gangsters. Hagen marries an Italian, Theresa, and has two sons, Frank and Andrew.
After longtime consigliere Genco Abbandando is diagnosed with terminal cancer, Tom becomes acting consigliere, succeeding to the post formally after Abbadando's death. Vito is initially reluctant to give Hagen the post full-time, considering that he is not Italian.
After Connie Corleone's wedding, Vito dispatches Hagen to Hollywood in order to persuade Jack Woltz, a big-time movie producer, to give singer/actor Johnny Fontane (Vito's godson) the lead role in his new war film. When Hagen first approaches Woltz, he offers help with some union trouble. Hagen then tells Woltz that one of his actors has moved from marijuana to heroin; a deleted scene in the movie shows that this information was going to be used to expose the star and thus hurt Woltz's studio. Woltz at first angrily refuses, but becomes more cordial once he finds out whom Hagen works for. Woltz invites him to his palatial house for dinner, and shows him his prized stud horse, Khartoum. During the dinner, Woltz tries to work out another deal with Hagen, but in the end refuses to cast Fontane, who had slept with one of his protégées, and angrily tells Hagen to leave his house. Hagen complies, but men working for the Corleones break into Woltz's stables and decapitate Khartoum, placing the horse's severed head in Woltz's bed. The next day, Hagen receives a call from a ranting Woltz, who threatens to bring the law down on the Corleones' heads; Hagen responds nonchalantly and hangs up. Shortly afterward, Woltz realizes that with Vito's connections, he is unlikely to win in court and will likely be murdered himself. He thus gives in to the Corleones' wishes and casts Fontane in his new war film.
Hagen next sets up a meeting between Vito (who is accompanied by underboss Sonny and caporegimes Peter Clemenza and Sal Tessio) and drug lord Virgil Sollozzo, where Sollozzo requests Vito's help in financing and protecting his drug business. Sollozzo raises his glass in respect to Hagen for having discovered that he is under the protection of the Tattaglia Family, a rival to the Corleones. Vito ultimately rejects the deal, however.
That December, Sollozzo and his bodyguards kidnap Hagen. At an undisclosed location, Sollozzo informs Hagen that Don Corleone has been shot and killed, and tells Hagen to persuade Sonny, who is running the family in his father's absence, to go along with the original deal. Hagen promises to calm Sonny down, but warns Sollozzo about an inevitable reprisal from Luca Brasi, the Don's fanatically loyal bodyguard and hitman. Unbeknown to Hagen, Brasi has already been killed by Sollozzo and Bruno Tattaglia. The meeting is interrupted when Sollozzo receives word that Don Corleone survived the shooting, ruining Sollozzo's plans. While Sonny initially appeared receptive to the Sollozzo deal, Sollozo knew that he would not listen to any deal while his father was still alive.
While he loves all the Corleones, Hagen is especially fond of Sonny and considers him a true brother for helping him as a child, and blames himself when Sonny is murdered by the Barzini Family. Vito goes into semiretirement in 1954, and his youngest son and new heir apparent, Michael, becomes operating head of the family. On his father's advice, Michael removes Hagen as consigliere in favor of having his father take the position on an informal basis, restricting Hagen to handling the family's legal business in Nevada, Chicago, and Los Angeles. Michael and Vito explain that the Corleones might have a fight on their hands with the planned move to Nevada and they need a "wartime consigliere". Though hurt, Hagen accepts the decision and remains loyal. In truth, Michael and Vito have been planning to wipe out New York's other dons to avenge Sonny and establish the Corleones' supremacy — an operation they have kept secret even from Hagen. In the book (and in a deleted scene from the film), Hagen asks why bodyguard Rocco Lampone has been secretly promoted to caporegime and why hitman Al Neri reports directly to Michael and not through Clemenza and Tessio. Neri and Lampone will play a key role in the assassination of the rival Dons that Michael is planning.
After Vito's death, Michael gives Hagen back his old post as consigliere. Hagen is present when Tessio is taken away to be executed for betraying the family and is also present when Connie's husband, Carlo Rizzi, is executed for setting up Sonny's death seven years earlier.
When Michael's wife, Kay, finds out that Michael ordered Carlo's death, she flees to New Hampshire. Michael dispatches Hagen to New Hampshire to persuade Kay to come back.
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