|This article is about a/an kaijin in Kamen Rider Black RX.|
A green and white-furred shaman with bat wings for ears. It's armed with a skull-headed staff and bananas that can be used as weapons as well as for other purposes. Byakki used his bananas to infiltrate a gold depository to steal the gold. Joe told Kohtaro about a weird old man who bought bananas from his banana stand after following him to his house. Shigeru and Saburo stumbled into his house and were captured. When Shunkichi learned about the weird old man from Kengo, he arrived to try to find his son only for Kohtaro to show up. When Shunkichi refused to let Kohtaro do it, Joe Punched him out. Kohtaro then found Shigeru and Saburo as the weird old man revealed his true form of Bya Fiend and attacked him. Byakki threw a banana into Kohtaro's chest knocking him out and then kidnapped him. Atop a building, Byakki tried to use a big banana to push the impaled banana further into his chest, until it broke. Kohtaro then revealed himself to be alive as he used his Robo Rider ability in a spit second transformation, which made it look like the banana to pierced him. Kohtaro transformed into Kamen Rider Black RX. After some banana attacks from Bya Fiend, RX became Bio Rider. While on Mach Jabber, Bio Rider attacked Byakki and used his liquid powers to evade his banana attacks. Bio Rider destroyed Bya Fiend's Skull Staff before destroying him with his Bio Blade.