Antagonist
Character Role Analysis
Sansón Carrasco
When we first meet the university grad Sansón Carrasco, Cervantes tells us that he has "all the signs of a malicious disposition" (2.1.3.2), which definitely sets him up as an antagonist. He also fills the role of antagonist nicely when he challenges Don Quixote to direct combat and, after losing, swears vengeance on the man from La Mancha. Of course, Carrasco gets a lot more sympathetic as the novel goes on, so we can't say that he's an all-out villain. Like the other characters in the book, he's pretty complicated, with both good and bad qualities.
Now, if there were a second antagonist in this book, it would have to be the dude who wrote the knock-off sequel to Don Quixote, Part 1, whom Cervantes spends many pages insulting in his own version of Don Quixote, Part 2. In fact, Cervantes actually alters the storyline of his own book just to spite the jerk who knocked him off. Now that's commitment.