You're right but to expand on it, protagonist assumes the main character from a positive light. The "hero" even if it's a bad guy is the protagonist (like the movie Megamind) the antagonist could also be a main character and generally assumes the negative role. The "villain." Of course it's not always that simple, but I hope this helps.

Protagonist is the correct word. It sounds fine! It can also be the main character in a book or a play.

Ogden was one of the earliest protagonists of educational reform. (example) This word also refers to a forerunner, frontier, or an advocate.

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It sounds fine. I would use it myself. If I were talking to, say, teenagers I might use the simpler and more ordinary word "hero." I agree with Ceej but I would put a slightly different spin on it: the protagonist is the character you identify with and cheer for--you want the protagonist to succeed. How about that? I just looked it up in ahdictionary and the original literal meaning, the "prot-" part, is "in Greek drama, the FIRST character to engage in dialogue with the chorus."