Yangchen

History
Avatar Yangchen was the last Avatar incarnate born out of the Air Nomads before Aang. Yangchen was a very wise Avatar, during her brief meeting with Aang, she convinced Aang not to feel so guilty, seeing that previous Avatars also struggled and made mistakes. Yangchen also imparted with Aang the purpose behind why the Avatar is born human: the Avatar must be compassionate to all people, and the only way of doing so is living among them. If the Avatar was some all-powerful spirit living at the top of a mountain, he wouldn't have anything in common with ordinary people. She acknowledges that the Avatar must experience human life and emotions, such as sadness, anger, joy and happiness, the Avatar must learn how precious human life is so he will see why he must do anything to protect it, and that with every incarnation, the Avatar Spirit comes closer to fully comprehending what it truly means to be human. By the end of the meeting, Aang learns that the Avatar Spirit has been healed, although his ability to enter the Avatar State has been disrupted, so he'll now have to rely on his friends rather than his past lives. Avatar Yangchen bids him farewell, saying that his journey in the Spirit World, reconnecting with his past lives will be like a forgotten dream.

Trivia

 * When abruptly asked for name of the female Airbender Avatar at the Pacific Media Expo, Bryan Konietzko (jokingly) dubbed her "Avatar Susan", indicating that they intially didn't plan on developing the previous Avatars aside from the two already introduced.