In middle school English classes, we learn that “protagonist” is a fancy word for “main character” or “good guy.” In film terms, this isn’t always the case.
The protagonist is the person who changes over the course of the story.
Often, this is the main character. It is very common for this person to be a good guy, too. But neither of these have to be true.
In Charlie and the Chocolate Factory, for example, Charlie is the main character, but Willy Wonka is the protagonist.