Jōji Yūki is the civilian identity of the character known as Riderman. He was originally a promising young scientist working for Destron, a sinister organization he genuinely believed was dedicated to creating a peaceful future. This belief stemmed from his childhood, as he was orphaned and subsequently raised by Destron’s leader, whom he saw as a surrogate father. His talent and dedication made him a rising star within the organization, but they also bred jealousy in his superior, the high-ranking general known as Marshal Armor.

Fearing that Yūki’s abilities threatened his own position, Marshal Armor framed the young scientist for treason. The punishment for this false crime was severe: Yūki was subjected to a torturous execution attempt in which he was lowered into a vat of acid. The process failed to kill him, but he lost his right arm in the process. Rescued by his loyal fellow scientists, Yūki underwent an impromptu surgery to survive. He was fitted with a prototype prosthetic right arm of his own design and donned a reinforced battle suit, transforming himself into the anti-heroic fighter known as Riderman, the "Avenging Demon".

Driven initially by a powerful desire for revenge against Marshal Armor, Yūki gave no thought to the nature of Destron as a whole. His sole focus was clearing his name and destroying the man who had ruined his life. In this quest, he saw Kamen Rider V3 as an obstacle, someone who interfered with his personal mission. However, as he clashed with V3, he began to understand the truth about the organization he had once served, realizing its global scale of evil. This revelation prompted a profound change in his motivations, and he abandoned his self-serving vendetta to ally with V3 in the fight for humanity’s freedom.

Because his transformation was a rushed procedure to save his life, Riderman is fundamentally different from other Kamen Riders. Aside from his prosthetic arm, his body is largely that of a normal human. His reinforced suit enhances his physical capabilities, granting him leaping ability about ten times that of an ordinary person and enough protection to withstand bullets and harsh environments, but his raw power is significantly inferior to that of V3 or other full-body cyborgs. To compensate for this weakness, Yūki relies on his intelligence, persistence, and his most distinctive feature: the Cassette Arm. This multi-functional prosthetic allows him to swap various attachments onto his right arm to suit different combat situations. The most commonly used attachments include the Power Arm, a large pincer-like blade for hacking through obstacles and enemies; the Rope Arm, which fires a hook-tipped rope for climbing, swinging, or entangling foes; the Drill Arm, a powerful rotating bit useful for infiltration; the Swing Arm, a flail-like mace on an extending chain; and the Net Arm, which can fire a sturdy net to capture opponents. The replacement process has a critical weakness, however, as it leaves him defenseless for approximately three seconds while the cartridge is swapped.

Throughout the story, his key relationships evolve dramatically from hatred for Marshal Armor to a deep, trusting alliance with Shiro Kazami, the man who transforms into Kamen Rider V3. He also remains loyal to the three scientists who saved his life after his accident. In a climactic act of heroism, Yūki sacrifices himself to prevent Destron from launching a devastating rocket to destroy Tokyo. Boarding the rocket, he detonates it in mid-air, seemingly dying in the explosion. In recognition of his courage and his crucial role as a fighter for justice, Kamen Rider V3 posthumously honors him with the title of Kamen Rider Number 4.