While he's in Sicily, Michael Corleone starts a new life, marrying Apollonia Vitelli (Simonetta Stefanelli). He falls in love instantly, as if struck by a bolt of lightning.
He courts her in the old-school Italian manner, letting her whole family chaperone their dates. We don't really learn all that much about Apollonia but we get to see that her death in a car crash changes Michael. It probably makes him a bit more ruthless (though he was capable of righteous revenge before).