I've learned new words! Hooray!

Um... I can't really say. Sora's obviously the protagonist. The deuteragonist could be either Kairi or Riku or Mickey. I'd lean towards Mickey myself because he's always been good, while Riku's more of an anti-hero. Finally, the tritagonist is probably Kairi.

Of course, Goofy and Donald are very important, but for the overarching story of the entire series, that's who'd I go for.