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 while in the Avatar State. The young airbender saw him again briefly in a similar vision during the Battle at Wulong Forest, this time among a great number of Avatars all standing in a line, after regaining his ability to enter the Avatar State.
Appearances in Korra's life
This 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 with her spiritual self.
While Unalaq, merged with Vaatu, was attacking Raava, Korra envisioned the fire Avatar among the Avatar lineup she had seen prior to regaining her bending; however, his connection to Korra was severed due to Unalaq's assault on and subsequent destruction of the Light Spirit.