Yangchen's predecessor

The unnamed fire Avatar hailed from the Fire Nation centuries prior to the Hundred Year War.

Background
While nothing has been revealed about his life nor his struggles as the avatar, the only confirmed fact about the unnamed fire Avatar is that he preceded Avatar Yangchen. At one point of his life, he erupted multiple volcanoes at once. After his death, the Avatar Spirit reincarnated into the Air Nomads in the identity of Yangchen.

Appearances in Aang's life
During a retreat to General Fong's fortress, Aang inadvertently entered the Avatar State as a result of his anger toward General Fong, who endangered Katara in an attempt to force Aang into the state.

Amidst the situation, Aang was taken by Avatar Roku on a journey to the Spirit World, where he was taught about the Avatar State and the Avatar Spirit in general. While explaining the extent of the Avatar State's capabilities, flashbacks of previous avatars empowered by the Avatar State emerged, where the unnamed fire Avatar was briefly shown erupting four adjoining volcanoes with the use of enhanced firebending and earthbending.

The unnamed fire Avatar made another appearance during the battle at Wulong Forest, where he was seen in Aang's vision alongside a line of Avatars when Aang unlocked the seventh chakra and immediately going into the Avatar State, resulting from an impact with a jutting rock.

Relatives

 * Yangchen (immediate successor)
 * All other Avatars (past lives and future lives)

Abilities
Albeit being shown only briefly, it is evident that the unnamed fire Avatar had complete control of the Avatar State, able to enter and exit it at will. He was seen controlling a number of small volcanoes with the use of his abilities, though the exact extent of his power remains unexplored.