There's an official webgame that takes place between Book 2 and 3, and goes more into the previous four Avatar's lives. If also sheds more light on the previous Water Tribe and Air Nomad Avatars, who you don't really get much from outside of the finale a bit.

There's also a few brief behind-the-scenes segments that focus on the reasons that the four primary bending styles are based on a specific real-life martial art. IIRC, Bagua was Air, Hung Gar was Earth, Tai Chi was Water, and Northern Shaolin was Fire. Obviously some characters have their own styles, like Toph's being based on Northern Mantis Style.

Watching Avatar the last airbender. Why are the fire nation evil/the bad guys?

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