Furious Demon God Gai is the leader and the most evil of the eastern Ashu Tribe, he is a purple and green demon who can conjure forth a bazooka-like weapon. Gai is a brash and savage demon who loves to destroy, laughing at the sight of others suffering.
Gai and BoukenSilver are bitter enemies, because long ago he told a young Eiji that he was part Ashu and that his mother was one as well and because he killed Eiji's father, Karato, when he tried to prevent Eiji's Ashu side from growing. When Gai saw Natsuki Mamiya's bracelet he said it was "The Proof of the Chosen One," and he and Hyouga kidnapped Natsuki and used her blood to free Grand Beast Rei from the 100-Demon Mirror. He used Rei's illusion-inducing powers to pose as Satoru Akashi's old companion Masaki to confuse and kill him with the Fighter's Crossbow, however when Satoru confronted his illusion it disappeared.
Though defeated by Eiji (who threw his monk staff at him like a javelin), and terminated by Ultimate DaiBouken and the Ashu Spirit Extinguishing, Arch Priest Gajah resurrected him as Quester Gai. In this form Gai wields two guns that also turn into short swords. But in either form, Gai was still Eiji's mortal enemy.
After the Quester's most powerful creation, the Homunculus was destroyed, Eiji and Gai faced each other in a final showdown. In the battle, Eiji finally kills his mortal enemy once and for all, but makes peace with him by building him a grave in memorial.
Gai's Ashu form was based on Gosei Sentai Dairanger's Mythical Chi Warrior Ryuuseioh; his Quester form is modeled after Dairen'oh.
Unfortunately, he revived again in Kamen Rider × Super Sentai: Super Hero Taisen together with the other villains of the past Super Sentai and Dai-Zangyack Great Commander. He and his partner Rei, as well as the rest of Dai-Zangyack teamed up with Dai-Shocker to fight the army of Kamen Riders and Super Sentai, including his enemies: the Boukengers, except his real enemy BoukenSilver. He is seen fighting HououRanger and Spade Ace.