Wan’s story is told in the second season of “The Legend of Korra,” known as “Book Two: Spirits.” Korra learns of his story while being healed by the sages of the Fire Nation after washing ashore there injured and suffering from amnesia. She learns how Wan lived 10,000 years before, stealing food for him and his friends in order to survive, and how he was eventually banished from his community and left to explore the wilderness.
On his journey, he inadvertently aids the spirit of darkness Vaatu in his battle against Raava, the spirit of light and peace, and so travels with Raava around the world to gain strength to aid her in her battle with Vaatu. The pair merge together, allowing Wan to become the first Avatar and Vaatu to be sealed away, but Wan proves unable to undo the cycle of destruction started by Vaatu and bring peace to the world in his lifetime, beginning the cyclical reincarnation of Avatars that leads eventually to Korra.
It’s a significant part to play in the story, covering a lot of ground over the course of just two episodes. Wan has to go from street urchin to powerful bender of the elements and bridge between the human and spirit worlds. Yeun handled it masterfully, according to IGN, which opined that the actor “effortlessly voiced both the childlike mischief and inherent wisdom that comes with being the Avatar.” No wonder the role would prove to be such a promising start for Yeun.