|“||"Farewell, V3. I leave the rest up to you. I'll detonate the Pluton Warhead somewhere safe.||„|
|~ Riderman to V3|
Sometime during his youth, Joji Yuki was abandoned and was considering suicide when The Great Leader personally intervened and adopted him. Yuki considered him his savior and willingly swore allegiance to Destron. He served loyally for Destron's cause as top scientist until a jealous Marshal Yoroi framed him for being a traitor of Destron to cover up his failures of defeating Kamen Rider V3 . As a result, he was sentenced to be tortured to death by being dipped into a pool of acid, which resulted in the loss of his right arm.
At first, he only sees Marshall Armor as his enemy and not the Destron Organization itself; furthermore, to prove his innocence to Destron, he seeks out to defeat Kamen Rider V3. However, he learns the truth about Destron and eventually joins Kamen Rider V3's cause. Even after joining V3 his devotion to The Great Leader was so strong he was willing to sacrifice himself to protect him by jumping in the way of V3's Fatal Kick. When asked by Kazami why he did such a thing he revealed his past relationship with the Leader and admitted that he simply couldn't let his savior be killed right in front of him. He pledges himself to V3's mission after this. Near the end of the series, he supposedly died in a Pluton Rocket explosion. Kamen Rider V3 cried out that he is Kamen Rider #4.
In reality, he manages to survive. In the Kamen Rider Spirits manga, it explains how he survived the explosion and landed in Tahiti where he lost his memory, but regained it when fighting a crab kaijin who was really a resurrected Marshal Yoroi.
Stats & Cassette Arms (or Attachment Arms)
- Punch : 1 ton
- Kick: 2 tons
What Riderman lacked in technological strength of V3, as well as the Riders before him, Rider 1 and 2, he made up for in persistence, smarts, and his wide array of prosthetic weapons to meet his needs. His weapons also were the basis for the "Rider Slash" and "Rider Shooting" finishing attacks.
- Rope Arm: Riderman's Default/Main Cassette Arm, which is used in various ways: as a means of escape, climbing, and in combat. The arm itself has modified attachments for Yuuki to use and adjust to various situations. There is his default Hook Arm, with hook attachment, for climbing and combat. When the mace end is attached, he gets his Swing Arm
- Swing Arm: A extension of the Rope Arm, which can be extended and used as a flailing weapon.
- Net Arm: A extension of the Rope Arm, where Yuuki's arm can shoot out a net to capture and restrain enemies (by Kamen Rider Spirits, it appears that this net is fireproof).
- Sickle Arm: A unused extension of the Rope Arm, which has been seen in various promotion; and yet, wasn't used in the series. Based on the photos, it is assumed it is simply used for combat.
- Power Arm: Riderman's strongest weapon, which increases Yuuki's attack in battle. Its primary use is to hack away at both obstacles, such as doors, and opponents. This weapon could've been easily used as a finishing attack, according to some fans. This weapon is the first instance of a Rider using a blade against a monster, which would later be a frequent armament in the Heisei Era-Riders.
- Drill Arm: Another weapon could've been easily used as a finishing move. It was only seen being used as an infiltration tool. It must be plugged in order for it to work best, although it will still work otherwise (by Kamen Rider Spirits, it appears to work just fine without external power).
- Machine Gun Arm: This weapon was only seen in the Kamen Rider Spirits manga. It was used to destroy the revived Marshal Yoroi in the manga, which had the appearance to be a regular machine gun. This weapon is the first instance of a Rider using a gun against a monster, which would later be a frequent armament in the Heisei Era-Riders.
- Spray Arm: Currently, it is only an unused concept. Its function was described as a weapon which sprays gas or smoke to distract adversaries.
- Octopus Arm: Currently, it is only an unused concept. Its function is described as a climbing tool, each finger on the hand resembling octopus tentacles.