Keiichirō Tokugawa was raised by his adoptive grandmother, Otomi, working at Nagasaki's Cherry Blossom Baths, unaware of his true heritage as the Tokugawa Shogun's son, separated from his biological mother Keiko in childhood.

He burned with a hot-blooded, combative spirit, filling his days with street fights while aspiring to become Japan's strongest man. This drive intertwined with a unique physiological condition: physical contact with women triggered severe hives, enforcing involuntary celibacy. He saw his virginity as a burden, ignorant of its deeper meaning.

His life shifted dramatically when assassins hunted him following the previous Shogun's death. During the attack, he discovered his ability to pilot the ancient biomechanical weapon Susanoo, an Onigami entombed beneath Nagasaki. This power was intrinsically linked to his virgin status, requiring a male virgin as the primary pilot. Activating Susanoo's combat modes further demanded a female virgin sub-pilot.

After repelling assassin Hōkōin with ninja Kiriko Hattori's aid, his grandmother unveiled his lineage as the Tokugawa heir. She charged him with reclaiming Edo from the rebel Shigeyoshi Hitotsubashi, controller of the rival Onigami Takemikazuchi.

Journeying toward Edo with allies including Kiriko, the fox spirit Chiharu, and mechanically gifted friend Hyakusuke, his relationships evolved through conflicts against sky pirates and the Shinsengumi. Chiharu's developing romantic feelings further complicated his condition.

His mastery of Susanoo grew through battle, achieving transformations like "Beast God Mode" and "Heavenly Thunder God Mode" during the final clash against Shigeyoshi. Yet, the full significance of his virginity and the ultimate resolution of his quest remained awaiting resolution at the series' conclusion.