Kagen is a Quartzer who has the power to transform into Kamen Rider Zonjis and one of the two secondary antagonists of the movie Kamen Rider Zi-O: Over Quartzer.
Kagen and the other Quartzers conspired to remake the Heisei era of Kamen Rider with themselves as the Kamen Riders. To accomplish this, they used Sougo Tokiwa/Kamen Rider Zi-O to unify the worlds of the Heisei Riders into a singular timeline for them to destroy and recreate anew.
In order to get Sougo to meet Krim Steinbelt and obtain the last of the Ridewatches, the Drive Ridewatch, Kagen and Jogen were sent into the Sengoku Era to target Krim's ancestor, Clara Steinbelt, to get Sougo to travel back in time. While Sougo was guarding Oda Nobunaga during the fight between the Oda and Takeda armies, Kagen ambushed them alongside several assassins from the Takeda clan and transformed into Kamen Rider Zonjis to battle Zi-O, but was fought off.
After Sougo gathered all of the Heisei Ridewatches, Kagen and Jogen returned to the other Quartzers as Woz delivered the Ridewatches to their leader, SOUGO Tokiwa. After Sougo is defeated and imprisoned, the Quartzers commence their plan to erase the Heisei Era by having the Dai Mazines form a portal and suck up the world of the Heisei Riders.
However, Sougo is freed by Tsukuyomi and Gou, and goes to assist Geiz in confronting the Quartzers. Kagen and Jogen assume their Rider forms to deal with Geiz and Sougo. After Woz defected from the Quartzers and rejoined Sougo's side, Zonjis and Barlckxs fought Geiz and Woz while Zamonas and the Dai Mazines dealt with Zi-O. Zonjis and Zamonas later fought the 19 Heisei Riders summoned by Sougo and were defeated when the 20 Heisei Riders all performed their Rider Kicks on them. As he was seemingly destroyed, Zonjis dropped his J Ridewatch, which was picked up by Barlckxs to use to grow giant.
However, after the Quartzers and their army are defeated, Zonjis is revealed to have survived with two Kasshin and vows to remake the Heisei Era himself. A new Rider then appears to stop Zonjis and announces himself as the first Reiwa Rider, Kamen Rider Zero-One. Tossing off his mantle, Zonjis declares he will crush the Rider and attacks him. Using Barlckxs' RoboRider Ridewatch, Zonjis fires missiles from his chest at Zero-One. However, Zero-One assumes Flying Falcon form and dodges the missiles before destroying Zonjis with a Rider Kick.
- Kamen Rider Zonjis's design is based off of the Kamen Riders from the three Direct-To-DVD Kamen Rider movies released by Toei in the 1990s, Kamen Rider Shin, Kamen Rider ZO and Kamen Rider J, with his name being a combination of all three Rider names.