Yangchen's predecessor

The unnamed fire Avatar was the Fire Nation Avatar preceding Avatar Yangchen.

Appearances in Aang's life
The unnamed fire Avatar first appeared to Aang in a spiritual vision directed by Avatar Roku, in which he was lavabending the magma from several volcanoes. The young airbender saw him again in a similar vision during the battle at Wulong Forest, this time among a great number of Avatars all standing in a line.

Appearances in Korra's life
The unnamed fire Avatar was present along with all the other Avatars when Aang passed on the knowledge of energybending to Korra and restored her bending, representing the newly established connection she had made to her spiritual self.