With a book as confusing as this one, it might be tough to pick out one lone antagonist. But if we have to (which we don't, but we will anyway), we'll go with the trickster translator, Ermes Marana.
After all, you could definitely argue that the whole plot unfolds entirely because of Marana, whose shenanigans at the publishing house have in some way caused all of your interrupted readings. His false translations and counterfeit books create a web of deceit that spreads outward, and the truth has become so lost in his giant shell game that you wouldn't even recognize it if you saw it. It's kind of exhausting, actually.
At first, it seems that Marana has posed as a translator so he can make some quick cash. But as the book unfolds, you learn that he has actually founded a group called the Organization of Apocryphal Power that produces counterfeit books all over the world. (Take that, Dan Brown).
Marana basically wants to show people that "behind the written page is the void: the world exists only as artifice, pretense, misunderstanding, falsehood" (19.18). In other words, he wants to undermine your belief that there is some sort of truth or meaning in books. Deep… we guess.
Simply put, the guy's confusing you for the sake of confusing you.
In response to the charge that he's mixed up a bunch of authors' names and books, Marana replies:
"What does the name of an author on the jacket matter? Let us move forward in thought to three thousand years from now. Who knows which books from our period will be saved, and who knows which authors' names will be remembered?" (9.75)
Actually, the dude kind of has a point. But that doesn't mean that Calvino wants us to like Marana.
Everything Marana does, every move he makes, is supposed to create uncertainty and undermine the pleasure you expect to get from reading. He is like a Norwegian trickster god (or better yet, an adolescent kid), playfully messing up the regular order of things just for disorder's sake. He even seems to mess with the normal flow of time, as you notice while reading his letters:
The chronology is also uncertain: there are letters that refer to previous communications, which, however, prove to have been written later (11.10).
Huh? If you came to If on a winter's night a traveler expecting something normal, trying to read this section of the book will probably be the most frustrating part of your experience. Calvino: 1, Readers: 0.