The protagonist or main character in Twilight is Bella. The story is primarily told from her point of view and the audience struggles along with her as she makes decisions about her relationship with Edward and the rest of the Cullens. One could argue that Edward and the rest of his "family" also play a protagonist role as many of the central events revolve around their roles.
James is definitely an antagonist. His desire to hunt down Bella and his attempt to kill her places him in direct conflict with her protagonist role.
Edited to say that protagonists are often referred to as "good guys" while antagonists are often referred to as "bad guys." While that does not always prove true, in Twilight it does.
The protagonist is the most important character in the story. In this text, that's Bella! We want her to adjust to her new life in Forks, we want her to fit in, we want her to find out what is going on wth Edward, and to overcome all of their differences and be together.
In the first book of the series, it would be easy to assume that the antagonist is James. He stands in the way of Bella and Edward. However, you really have to think about the forces fighting against Bella, and that is either we humanity or Edward's vampirism. If they were both humans or both vampires, they would have virtually no obstacles and would be able to be together. But it's their differences keeping them apart. Therefore, you could argue that Bella's humanity, since she wants to be a vampire, is the antagonist.
To use the terms "good guys" and "bad guys" is a incorrect becuase a "bad guy" could be the protagonist and a "good guy" could be the antagonist. Using those two terms as a means to help students tell the difference between protagonists and antagonists is pretty much telling them that a book written in James' perspective makes James the antagonist.
The protagonist is the character that is telling the story. They are also refered to as dynamic or round characters. They are characters that change throughout the novel, play, etc.
The protagonists are Edward, Bella, and Jacob. The antagonist is James.
The protagonist is Bella and the antagnoist is James