A lot of people have the misconception that a protagonist is only the main character of the book. However, that is not true. A protagonist is any character in which the events happen to, for, or against them. They must also be dynamic, meaning they are not the same person at the end of the story as they were in the beginning. Keep in mind that a protagonist does not always have to be a good guy. This character could be good or bad. They just have to meet the above requirements.
An antagonist is a pretty straightforward character type. This character goes against the protagonist. They try to prevent the protagonist from achieving goals. The antagonist is also a static character, meaning they do not change at the end. Just as the protagonist can be a good or bad guy, so can the antagonist.