The Vicomte of the Opera
Le Fantôme de l'Opéra without Raoul would still be an intensely gripping and tragic story. Le Fantôme de l'Opéra (Le Vicomte de l'Opéra, perhaps?) without Erik, however, would be quite bland, at least by comparison. (Think about it – a young, wealthy vicomte falls in love with his childhood sweetheart when he sees her perform at the Opéra, and after several months of courtship, he marries her, against the wishes of his older brother. The end.)
(This being the reference to which my response was: "I already wrote it, as Count Philippe Takes a Hand" :-D)