The Spirit of Fire is an elemental spirit

Elemental

An elemental is a mythological being first appearing in the alchemical works of Paracelsus in the 16th century. Traditionally, there are four types:*gnomes, earth elementals*undines , water elementals*sylphs, air elementals...

. In the Manga

Manga

Manga is the Japanese word for "comics" and consists of comics and print cartoons . In the West, the term "manga" has been appropriated to refer specifically to comics created in Japan, or by Japanese authors, in the Japanese language and conforming to the style developed in Japan in the late 19th...

 and Anime

Anime

is the Japanese abbreviated pronunciation of "animation". The definition sometimes changes depending on the context. In English-speaking countries, the term most commonly refers to Japanese animated cartoons....

 series Shaman King

Shaman King

is a Japanese manga series written and illustrated by Hiroyuki Takei. Shaman King follows the adventures of Yoh Asakura as he attempts to hone his shaman skills to become the Shaman King in the Shaman tournament....

it is primarily associated with Asakura Hao, its Shaman.

Background

The Spirit of Fire is one of five powerful elemental spirits known as the Five Grand Elemental Spirits (Godaiseirei in Japanese). They are derived from the Great Spirit itself, and like it, constitute a collective of souls. They can be controlled by their respective Elemental Warrior, or a shaman of extraordinary power (like Hao). The power of the Godaiseirei is unmatched by any other spirit, except for maybe the Great Spirit itself.
The Spirit of Fire has the ability to consume other souls and spirits in order to further enhance its own power. Its primary medium for its Over Soul is air, which allows Hao to manifest the Spirit of Fire with incredible speed. However, it also has the ability to transform itself into the different five elements

Five elements (Chinese philosophy)

The Wu Xing, also known as the Five Phases, the Five Agents, the Five Movements, and the Five Steps/Stages, are chiefly an ancient mnemonic device, in many traditional Chinese fields....

 through the doctrine of the five phases, which is made possible through Hao's mastery of said elements.

History

500 years before the start of the series, Hao was reincarnated as a member of the Patch tribe, the organizers of the Shaman Fight. Although he would fail to become the Shaman King, he succeeded in the stealing from the tribe the Spirit of Fire, which became his spirit ally when he was again reincarnated 500 years later. Its power was greatly diminished because of its journey back from Hell (where Hao negotiates the terms of his reincarnations with Taizan Kuzun, the god of the dead), forcing it to regain its strength by consuming other souls and spirits. Together, the two were an unstoppable force that was all but destined to win the Shaman Fight.

In the manga, Hao discards the Spirit of Fire after the other competitors forfeit the Shaman Fight, making Hao the Shaman King and the inheritor of the Great Spirit. The Spirit of Fire then became the spirit ally of Lyserg Diethel

Lyserg Diethel

Lyserg Diethel is a fictional British character from the anime and manga series Shaman King. He is an orphaned from a young age, when Asakura Hao killed his parents,when they refused to join him. This in turn makes him seek revenge, and eventually made him participate in the Shaman Fights to kill...

, its destined Elemental Warrior.
In the anime, Hao would ultimately deem the Shaman Fight to be a pointless contest, and would move to acquire the Great Spirits by force. After fighting through various obstacles, the Spirit of Fire would seemingly consume the Great Spirit itself, being reborn in a golden hue ten times its previous size.