Kuroki Gai lived on Earth as a compassionate pacifist, sharing a close bond with Shurato Hidaka. He consistently displayed kindness through actions like rescuing endangered animals and sharing food with Shurato. Both trained in martial arts under Shurato's grandfather, with Gai exhibiting superior fighting skills and greater self-control. During a junior martial arts tournament, Gai and Shurato were transported by a beam of light to Tenkūkai, a parallel world governed by spiritual energy called Sohma. Upon awakening there, Gai underwent a drastic personality shift, transforming from a gentle pacifist into a ruthless antagonist who immediately attempted to kill Shurato. In Tenkūkai, Gai was revealed as the reincarnation of Yasha-ō, one of the eight legendary warrior kings known as the Hachibushu. His shakti and armor manifested wolf motifs, and he wielded a longsword. His abilities included the Demonic Wolf's Gale Sword and Yaksha's Life Drainer. Gai's antagonistic role stemmed from manipulation by Indrah, Vishnu's advisor and a spy for the destructive Asura Gods. Under Indrah's influence, Gai framed Shurato and the warrior Hyūga for petrifying Vishnu. This deception turned the other Hachibushu and Tenkūkai's inhabitants against Shurato and Hyūga, igniting widespread conflict. Gai maintained his antagonistic stance throughout much of the series due to Indrah's corruption. However, once Indrah's influence ended, Gai's inherent compassion resurfaced. He ultimately returned to his original alignment, rejoining the forces of good. No concrete information exists regarding his involvement or development in the OVA series *Dark Genesis*.