Wan reveals himself to Korra as the original incarnation of the Avatar. I’m a little surprised that they didn’t try this in Avatar: The Last Airbender but nevertheless it was cool to see the original unchanged soul to be the very first avatar.
Wan wasn’t a very special adult, just a poor guy with a knack for attracting the wrong kind of attention but he was skillful enough to get what he wanted most of the time.
He eventually befriends the greatest light spirit and gets the power to bend fire, air, water, and earth one by one.
It seems that the Avatar state is really just his bond to the spirit cranked up.
He dies as a somewhat old man at the end of a battle. Agreeing with the spirit to be reincarnated over and over again to help bring peace to the world.