|This article is about a/an ally in Kamen Rider Build.|
Koji Hotei had been friends with Gentoku Himuro since childhood. They frequently attended the Manekineko karaoke bar when Gentoku was troubled. Shortly before the Sky Wall Disaster of 2008, the two young men vowed to enter politics to improve the nation of Japan. However, the Disaster saw the rise of the Sky Wall which split the island into three seperate nations: Touto, Hokuto, and Seito.
By 2015, Koji had become Aide to Gentoku's father Taizan Himuro, now Prime Minister of Touto, while Gentoku became director of the Touto Institute of Advanced Matter Physics. While Koji and the elder Himuro promoted peace, their rivals in Hokuto and Seito, whose leaders had been mentally disturbed by the light from the Pandora Box, the extra-terrestrial artefact which caused the Disaster before being recovered by Touto, built up their military forces for war. Gentoku, who had likewise been affected by the Pandora light, felt that Touto needed to keep up with their rivals and so urged Koji to convince his father to increase military power. Koji, however, insisted that diplomacy was the way to keep the nation safe.
Shortly after, Koji met with Gentoku at Manekineko, where he voiced his belief that, to avoid total destruction, Touto had no choice but to relinquish the Pandora Box and surrender to Hokuto and Seito. It was at this point that Koji revealed that Taizan Himuro had offered him the position of Prime Minister of Touto, revealing to Gentoku that he would soon have the power to enact his plan. However, shortly after leaving, Koji was murdered by "Soichi Isurugi", who left his fresh cadaver before Gentoku. While his reaction to the loss of his old friend was initially one of anger, Gentoku, growing increasingly unhinged, soon conceded that "Isurugi" had filled his wish by murdering Koji, leading him to make his "pact with the devil" and founding Faust.
Following Koji's death, Gentoku took his place as Aide to the Prime Minister.