Aang is a hero - a 12-year-old hero - who likes to stir up trouble. As the last Airbender and the only known survivor of the Air Nomads, Aang's destiny as the Avatar is to master all four elements - Air, Water, Earth and Fire. But he'd rather search for adventure than save the world.
Katara is a 14-year-old girl with a big heart who's determined to save the world - or at least never stop trying. Katara wants to live up to her potential and become a Waterbender in order to save her tribe, but she's still learning to control her powers.
Katara's brother Sokka is a typical teenager who believes his way is the only way. While he shows the strength and determination of a warrior, Sokka lacks the experience to use them effectively. Unlike Aang, Sokka doesn't go for the spiritual stuff and doesn't trust anything he can't see or touch.
Momo is Aang's pet winged-lemur. He's super smart and totally trainable, and is able to perform many human tasks. Momo has a very enhanced ability to hear and smell, allowing him to sense danger from miles away. But he's also a bit of a troublemaker.
Zuko is a big bully. He's a focused, driven teenage Firebender who sights are trained on only one thing: capturing the Avatar. Zuko is determined to capture Aang - which is easier said then done, even for a Firebender.