A green and white-furred shaman with bat wings for ears. It is 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 Kotaro about a weird old man who bought bananas from his banana stand after following him to his house. Shigeru and his friends stumbled onto his house and were captured. When Kotaro arrived and found Shigeru and his friends, the weird old man revealed his true form of Bya Fiend and attacked Kotaro. Byakki threw a banana into Kotaro'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 Kotaro used his Robo Rider ability, which caused the banana to break and allowed him to recover. Kotaro transformed into Kamen Rider Black RX. After some banana attacks from Bya Fiend, Kamen Rider Black RX became Bio Rider. While on Mach Jabba, Bio Rider attacked Byakki and used his liquid powers to evade his banana attacks. Bio Rider broke Bya Fiend's staff before destroying him with his Bio Blade.