A protagonist is the central character in a story: the protagonist of Huckleberry Finn is — guess who? — Huckleberry Finn.
A novel, movie, or play might have many main characters, but it can really only have one protagonist — or maybe two in the case of, say, Romeo and Juliet. That's because protos means "first" in Greek, and agonistes means "competitor" or "actor." It can also mean a leading figure in a real-life situation: "Abraham Lincoln and Jefferson Davis were the protagonists of the U.S. Civil War." Don't use it to mean "a supporter of an idea or cause"; the word you're looking for in that situation is proponent.