The whole idea of an "antagonist" isn't really applicable to Giovanni's Room. There aren't good guys and bad guys. There are just guys. Lots of guys. That said, the character who behaves in the most despicable manner is, without a doubt, Guillaume. Even as David narrates events, we have enough material to sympathize with Guillaume. He is an ugly old man, unloved by the men around him, and constantly taunted with things that he cannot have. Yet he also uses his wealth to leverage younger boys, to take advantage of them, and, in the case of Giovanni, to abuse and mock him.