|This article is about a/an duplicate character/rider in Kamen Rider Den-O.|
- Not to be confused with Kotaro Nogami, Ryotaro's grandson, who became New Den-O
When Ryotaro and Hana are trapped in the year 2000 by Kamen Rider Gaoh they encounter Ryotaro's child self. With Ryotaro lacking memories of being Kamen Rider Den-O due to being attacked by Gaoh and unable to transform, the young Ryotaro is given the nickname Kotaro and decides to go with them to stop Gaoh's plans. During a fight with ninjas in the Edo Period, Momotaros possesses him with his contract to Ryotaro extending to the past and transforms into Kamen Rider Mini Den-O.
After the battle Kotaro is brought back to his time but not before seeing his late parents and then bids the gang farewell.
Plat Form (プラットフォーム Puratto Fōmu) is Mini Den-O's most basic default form undersuit, which is briefly seen during Mini Den-O`s transformation sequence before the Aura Armor and the Denkamen facemask attaches.
- Den-O Belt - transformation device belt that allows for form switching.
- Rider Pass - a ticket system that allows for transformations as well as the operation of the motorbike Machine Den-Bird and Den-Liner
- DenGasher - a four-piece standard issue assemble side-arm that reconfigures to form various weapons.
As a result of an unknown force altering the present Ryotaro is regressed into an adolescent form. While investigating an error in time with Deneb and Kamen Rider New Den-O he encounters a child Yuto Sakurai. Later after finding three of the scattered Tarōs and going into the time of the battle against Oni Island Ryotaro reunites with Momotaros and fights alongside everyone.
Despite being a child, Ryotaro does not assume Mini Den-O, instead becoming an adult Den-O. This is possibly because unlike Kohana who was regressed into a child due to her future being recreated, Ryotaro was de-aged as a result of an unknown tampering of time.
- In I'm Born, unlike later Child Riders Wataru and Asumu, Kotaro does not grow to the size of an adult when transforming into his Rider Form, becoming Kamen Rider Mini-Den O.
- When Mizoguchi later portrayed the de-aged Ryotaro, he instead grew to an adult size when transforming into his Rider form, although by this time of production, Mizoguchi had grown into a young teenager.