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.
In the sixth volume of the manga, Kou makes the near-fatal mistake of challenging Goku to a death match for the sutra. Both sides receive heavy injuries, as Goku removes his limiter for the fight, releasing the unstoppable Seiten Tasei. Dokugakuji manages to get the injured prince home, but is forced to hand him over to the care of Nii Jianyi and his scientists. Kou is not seen again until the eighth volume, when he reappears healthy but brainwashed. Nii reveals that his mind was holding back his conviction, making him lack the power to defeat Goku.