Gai Kuroki originates as a compassionate Earthling, demonstrating consistent selflessness from childhood onward. He rescues distressed animals like a dove and a dog, shares food with his best friend Shurato during the latter's punishments, and trains alongside him in martial arts under Yutaro. Gai exhibits superior skill and self-control, fostering mutual respect and rivalry. During a tournament, both are transported to Tenkūkai, a spirit-governed realm. Gai immediately undergoes an unexplained shift, displaying aggression and attempting to kill Shurato due to brainwashing by Indrah—Vishnu’s advisor secretly serving the Asura Gods. Embracing his identity as Yasha-ō, the Yaksha King, he wields darkness-based powers through wolf-themed armor and techniques like Demonic Wolf’s Gale Sword and Yaksha’s Life Drainer. As a primary antagonist, Yasha-ō aids Indrah in framing Shurato and Hyūga for petrifying Vishnu, pursuing former allies with lethal prowess. His black armor with a red circle visually signifies his alignment. Fleeting moments of pacifism surface but are suppressed. After Indrah’s defeat, residual darkness keeps Gai antagonistic under Shiva’s influence. His redemption culminates in sacrificing himself to protect Shurato against Shiva, merging their souls to defeat her and accepting death as atonement. In the OVA *Dark Genesis*, Gai’s legacy persists through spiritual fusion with Shurato. His consciousness surfaces during critical battles as a darker aspect of Shurato’s abilities, exploring their unresolved bond. Physically, Gai has pale skin, white hair, and red eyes—accentuated in Tenkūkai. His wolf motif defines his armor and stealth-based combat style, contrasting his Earthly kindness with his corrupted Tenkūkai persona, underscoring a tragic arc resolved through self-sacrifice and unity.