Rai Washio

Rai Washio is one of the Namba children and a supersoldier of the Seito government. He is an antagonist in Kamen Rider Build.

History
Rai and his older brother Fu were raised by Namba Heavy Industries as Namba Children, being indoctrinated and undergoing brutal training to become soldiers.

After Kiba defeated some Seito Guardians as Owl Hazard Smash, Rai and Fu showed up and quickly dispatched the Hokuto Crow as Remocon and Engine Bro's. They then tell Grease to become Seito weapon if he wants Kiba to live.

After torturing Kiba for a bit, Kazumi and Akaba appear and confront the Washio brothers and free their comrade. Grease and Akaba manage to escape with Kiba, but Grease later suffers a shot from Fu's Nebula Steam Rifle. Rai's brother attempts to kill Grease, but Kiba takes the attack. Akaba and Kazumi then escape to Sky Road to meet up with Sento and Ryuga.

Rai and his brother pursue them and are fought by Sento and Ryuga as Build Rabbit Tank Hazard and Cross-Z Charge respectively. However, the battle is interrupted when Kamen Rider Rogue appears.

During Seito's forces incursion on the Touto government office, Rai and his brother provided cover for Rogue as he went to acquire the Pandora Box. The Washio brothers fought Kamen Riders, Cross-Z Charge, Grease and Build, though Build managed to catch up with Rogue.

The Washio brothers later represented Seito alongside Rogue in a three-stage Proxy Battle with Touto. Rai was defeated by Grease in the first round. During his fight with Cross-Z Charge in the second round, Fu got Ryuga to lower his guard by claiming him his brother would be executed if he failed, using Ryuga's hesitation as an opportunity to knock him out of the battle. Rai then appeared unharmed to gloat with his brother.