Recently went over this in my Children's Literature class
An antagonist is a character, group of characters, institution, or concept that stands in or represents opposition against which the protagonist(s) must contend. In other words, an antagonist is a person or a group of people who opposes a protagonist
If Clay is the protagonist, the plot of the show revolves around what problem the antagonist has created, in this case Hannah's death, so Hannah by that logic, is the antagonist. Antagonist doesn't mean evil like many people assume it does however, it just means the person who drives the conflict in the story, so yes, I would say Hannah Baker was the major antagonist
