Kay Adams (Diane Keaton) plays an even bigger role in The Godfather Part II. But she's still pretty important in the first movie—she becomes Michael's second wife, after all. While she's in love with Michael, Kay seems pretty conflicted. She knows that Michael's father runs a Mafia family, but he tells her that he's different… which he is, until he isn't. Also, Michael can't tell her that he loves her—maybe because he doesn't want to drag her into anything.
After returning from Sicily, Michael finally does marry Kay. But this time, their marriage is shadowed by the fact that Michael is maturing into an extremely ruthless leader of organized crime. How will Kay deal with this? The end of the movie darkly suggests that it'll be a source of problems to come, and The Godfather Part II follows up on this to stunning effect.