It's tough to say who's more of an antagonist in this book, Marana or Lotaria. On the one hand, it's the actions of Marana that have created the confusion that keeps you from finishing the stories you've begun reading. Oh, and Marana eventually emerges as a rival of sorts for the love of Ludmilla.
But Calvino seems to paint Marana with a much more sympathetic brush than he does Lotaria. Lotaria is an annoying and bossy student of literature, who seems to insist on only one thing: that a person should never take pleasure in reading. And isn't that the whole point of If on a winter's night a traveler?
But in the end, the place of antagonist goes more to Marana, because of the way his devious acts drive the overall conflict of the book.