Gai Kuroki begins as a compassionate pacifist from Earth, rescuing injured animals and sharing meals with his best friend Shurato Hidaka. He trains alongside Shurato in martial arts under Yutaro Hidaka, consistently defeating Shurato with superior skill and self-control. During a tournament, the goddess Vishnu transports them to Tenkūkai, revealing them as reincarnations of Hachibushu warrior kings. Shiva corrupts Gai during this transmigration, infusing him with dark Sohma to serve the Asura Gods, transforming his personality into cruelty and hostility. Reborn as the Yaksha King, Gai commands wolf-themed armor, a longsword, and chakram, wielding techniques like "Demonic Wolf's Gale Sword" and "Yaksha's Life Drainer." He allies with the traitorous Indrah—who frames Shurato for Vishnu’s petrification—and repeatedly attempts to kill Shurato. Later revealed as Brahma’s successor destined to share ultimate power with Shurato, Gai resists Shurato’s attempts to restore his original self until their final battle against Shiva. Mortally wounded, Gai breaks Shiva’s control, merging his dark Sohma with Shurato’s light Sohma to summon Brahma’s armor, destroying Shiva but sacrificing his life. Posthumously, Gai manifests as a spirit when Shurato recognizes universal balance requires both light and dark Sohma. His legacy endures through sister Mina Kuroki in the OVA "Dark Genesis," where her grief spawns the destructive entity Scrimil, endangering both worlds. Gai’s narrative embodies duality—first through his contrast with Shurato, later as the dark counterpart essential for cosmic equilibrium.