Joji Yuki is Riderman, the former lead scientist of Destron and a major antagonist-turned-anti-hero in Kamen Rider V3.
Joji Yuki was suicidal when he was found by the Great Leader, who prevented him from going through with it and adopted him. Seeing the Great Leader as his savior, Joji Yuki pledged loyalty to the Great Leader and Destron.
Kamen Rider V3
Joji Yuki served as the head of the Destron Science Division until he was framed for treason by Marshal Armor, who feared he was a rival for a higher position. Armor attempted to execute him by dipping him in a vat of acid until Yuki's followers in the Destron Science Division intervened and rescued Yuki before setting off a bomb and escaping.
Injured and having lost his right arm to the vat of acid, Yuki was converted into Riderman to save his life. Still loyal to Destron, Yuki at first only focuses on trying to kill Marshal Armor. He at one point tries to kill V3 to prove his loyalty to Destron. However, thanks to V3, Yuki eventually learns the truth about Destron's terrorism and turns against the organization. Despite this, Yuki could not shake his loyalty to the Great Leader, whom he still saw as a parental figure, and even prevents V3 from killing him once. Yuki would later seemingly sacrifice himself to save Japan from a Pluton Rocket fired by Destron.
However, Riderman survived the rocket's explosion and would return in later Kamen Rider series to assist the Kamen Riders.