Protagonist
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Character Role Analysis
Michael Corleone
You could try to argue that Vito Corleone is the protagonist, but Michael fit all the stages of our "Hero's Journey" section (or really "Anti-Hero's Journey," in this case).
Michael comes up against big odds and has to go from inexperience to experience, whereas Vito Corleone is already experienced—he's already made his way through the world and is now reaching the end. But Michael is the one who plays the biggest role in confronting and defeating his enemies, and changing in the process. Vito really functions more as a mentor.