He is a youkai prince who lives in Houtou Castle together with his father Gyumaoh and mother Ratsetsunyo.

He is one of the strong willed youkai who has the "heart of gold," excels in martial arts, able to emit fire projectiles and knowledgeable in youjutsu, including youkai summoning spells such as his famous Engokuki (Demon of the flame prison).

In Saiyuki, Kougaiji is the enemy of Genjou Sanzou and Sanzo Ikkou (Genjou Sanzou's party) as he is forced to work for his stepmother Gyokumen Koushou (whom he detests) to retrieve the 5 Sutras including the maten sutra from Genjou Sanzou. This is so that he can obtained a reward from Gyokumen Koushou who promised to revive his mother, Ratsetsunyo who has been placed under a curse which turned her into stone in Houtou Castle.

His strong devotion to his mother and kind nature earns him loyalty from his youkai followers and his companions who are willing to serve him in his task. This include his half sister Lirin, whom he cares about very much, his personal bodyguard and swordsman, Dokugakuji, who is more like an older brother to him, and his personal alchemist called Yaone, whom he saved from the clutches of the woman-eating Hyakuganmaoh.

(Source: Saiyuki Wikia)