Whether Aang is the main character or not is entirely up for debate. Personally, I'ver always thought of Avatar as having a parallel storyline with two different heroes (Aang and Zuko) on two different quests (Aang's quest to save the world, and Zuko's quest for redemption). Granted, their fates have brought them together for purposes such as Aang learning firebending, but by and large for the majority of the series they have been on seperate paths, both of which lead to a seperate "ultimate evil" that will have to be confronted and defeated (Aang vs Ozai and Zuko vs Azula). That's one of the things I love about Avatar; the multiple story lines. You don't have to care about all the storylines (although I do), you can just pick one that you love and want to focus on, and for the others you can just sit back, admire the depth of the storytelling, and enjoy the bending battles.