Joji Yuki is a character whose life is defined by a profound and tragic conflict between loyalty and justice. Once a promising young scientist, his journey transforms him into the hero known as Riderman, a figure driven by revenge who ultimately finds a greater purpose.

Yuki's past is marked by deep despair and salvation. Abandoned as a youth and contemplating suicide, he was rescued by the Great Leader of the evil organization Destron. Seeing the Great Leader as a savior and father figure, Yuki swore absolute loyalty to Destron and became one of its top scientists, genuinely believing in the organization's benevolence. This faith was shattered when a jealous superior, Marshal Armor, framed Yuki for treason to eliminate him as a rival for promotion. As punishment, Yuki was sentenced to death by being dipped into a pool of acid, an ordeal that resulted in the loss of his right arm before his loyal assistants helped him escape.

To survive his injuries, Yuki was transformed, becoming Riderman. His most defining physical feature is his prosthetic right arm, which can be fitted with various cassette-based attachments for combat and utility. His default attachment is the Rope Arm, which can be used for grappling, climbing, or as a striking weapon. Other known attachments include a Hook Arm, a flail-like Swing Arm, a Net Arm for capturing enemies, a Power Arm for enhanced strength, and a Drill Arm for infiltration. Unlike many of his fellow Kamen Riders, Yuki does not use a transformation belt; he becomes Riderman by simply donning his helmet, which reveals his cybernetic enhancements.

Initially, Yuki's personality is consumed by a singular, burning desire for revenge against Marshal Armor. He does not initially see Destron as his enemy, only the man who betrayed him, and he stubbornly clings to the belief that he can prove his innocence and clear his name. This single-mindedness leads him to challenge Kamen Rider V3, viewing the hero as an obstacle to his personal vendetta. Despite being outmatched, Yuki's persistence and selflessness are evident; he fights relentlessly, driven by a misguided sense of honor.

His role in the story evolves significantly through his interactions with Shiro Kazami, the man behind Kamen Rider V3. Through Kazami's words and example, Yuki begins to see the true, evil nature of Destron and the lives at stake. He eventually abandons his quest for mere revenge to become a genuine ally, joining V3's crusade against their shared enemy. However, his internal conflict does not end there. Even after turning against Destron, his deep-seated affection for the Great Leader, his adoptive father, persists. This emotional bond is so powerful that he once sacrifices himself to protect the Great Leader from V3's attack, still unable to let his savior be killed before his eyes. It is only when he fully comprehends the Great Leader's true, monstrous nature that he is finally able to wholeheartedly embrace his role as a Kamen Rider.

Yuki's development culminates in a selfless act of heroism. To stop Destron's final plan to destroy Tokyo with a Pluton Rocket, he pilots the missile itself, seemingly sacrificing his life in the ensuing explosion. In recognition of his ultimate courage and redemption, Kamen Rider V3 declares him Kamen Rider 4. Despite this apparent death, Yuki survives the blast and continues to appear in later adventures, joining forces with other Kamen Riders to face new and greater threats, solidifying his legacy as a true and enduring hero. Notable abilities include his genius-level intellect, which he applies to both scientific and tactical problems, and his unwavering persistence in battle, often compensating for any lack of raw power with his versatile prosthetic armaments.