Rai Washio

1="Jundou."

- Transformation phrase

2="I am Engine Bro's, and my brother is Remocon Bro's. Which one do you wanna fight first?"

- Rai introduces himself and his brother to the Owl Hazard Smash.

Rai Washio (鷲尾 雷) is the younger brother of Fu Washio, hailing from Seito. He is able to transform into the white Kaiser, Engine Bro's (エンジンブロス).

Character History
Little is known about Rai past, but he and his brother were raised by Juzaburo Namba institute as his 'Children'. There, Rai and Fu were indoctrinated and trained as soldiers.

Rai, along with Fu was first shown at Hokuto after Kiba defeats Seito Guardians as Owl Hazard Smash. They quickly defeat the latter as Engine and Remocon Bro's easily and taunt Grease to become Seito weapon, if he wants Kiba to live.

After they torture Kiba, Kazumi and Akaba arrives, and Kazumi rejects Rai's offer and fight him as Grease. Grease, along with Akaba and Kiba escapes, but later shot by Fu using Nebula Steam Rifle. Fu's about to kill Grease, if Kiba didn't took the bullet for him. Akaba later helps his boss escape once more, to Sky Road where Sento and Banjou awaits.

As Washio brother finally caught up to them, Sento and Ryuga fights them as Build Rabbit Tank Hazard (later Rabbit Tank Sparkling) and Cross-Z Charge respectively, until Rogue make his appearance.

Joining Seito's forces in an attack on the Touto government office, the Washio brothers covered Rogue as he went in to claim the Pandora Box, engaging Kamen Riders, Cross-Z Charge, Grease and Build before the latter caught up with Rogue. Eventually, the Kaisers succeeded in defeating the Sclashjelly Kamen Riders.

Alongside Rogue, the Washio brothers represented Seito in a three-stage Proxy Battle with Touto. Fu faced Grease in the first round and initially overwhelmed the Kamen Rider only to be shockingly defeated when Grease, recalling his deceased comrades, persevered and struck back at Engine Bro's. With Fu defeated, his Gear Engine was used by Rai alongside the Gear Remocon to face Cross-Z Charge as the combined Hell Bro's in the second round. During their battle, Fu claimed to Ryuga that, since his brother had failed, Rai had been taken away by Seito with his survival depending on the elder Washio's victory, using the resulting hesitation in Ryuga to knock him out of the contest. Soon after, Rai arrived, having been unharmed, and gloated with his brother.

Personality
Compared to his brother, Rai is more outspoken and seems to be prideful with his abilities. He had tendency to mock his opponent to let his/her guard down. He also didn't hesitate to play dirty like capturing an unconscious Misora and keep her hostage for Touto Kamen Riders to relinquish their Fullbottles. Nevertheless, both Fu and Rai are cruel and ruthless, and likes to torture their defeated opponents. Rai seems to dislike Gentoku, calling him Seito's Kamen Rider in disgust.

Forms
Engine Bro's "Gear Engine! Funky! (Rock music, engine revving) Engine Running Gear!"

- Transformation announcement

Statistics Ability Perimeters: One half of the Hell Bro's, Engine Bro's (エンジンブロス) is the white Kaiser, transformed into by inserting Gear Engine into the Nebula Steam Gun. This updated Kaiser System was created by Nariaki Utsumi, based on that originally developed by Kaisei Mogami.
 * Height: 203.0 cm
 * Weight: 103.5 kg
 * Punching Power: 30.2 tons
 * Kicking Power: 37.2 tons
 * Maximum Jump Height: 48.6 m
 * Maximum Running Speed: 100 m. per 1.7 sec.

Engine Bro's possesses strength to rival Kamen Rider Grease, Kamen Rider Cross-Z Charge, and Kamen Rider Build RabbitTank Sparkling Form, only beginning to struggle with Build's RabbitTank Hazard Form. Engine Bro's, however, is not without a weakness. Being based on an engine, he can easily be damaged with water-based attacks.

Engine Bro's has 3 different finishers:
 * Nebula Steam Gun finisher:
 * Funky Drive (ファンキードライブ): Engine Bro's channels the power of a Gear and performs a powerful blast attack.
 * Gear Engine:
 * Gear Remocon: Engine Bro's shoots energy gears at the enemy. This finisher can be accompanied with a Kaiser wielding the Steam Blade.
 * Funky Attack (ファンキーアタック) Engine Bro's channels the powers of a Fullbottle and performs a powerful blast attack.
 * Nebula Steam Rifle finisher:
 * Funky Shot (ファンキーショット): Engine Bro's channels the power of a Gear or Fullbottle and performs a powerful blast attack.
 * Gear Engine:
 * Gear Remocon:

Appearances: Build Episodes 23-26, 29-30, ROGUE

Devices

 * Nebula Steam Gun - Transformation device and personal weapon. Shares with Remocon Bro's.
 * Fullbottles - Transformation trinkets.

Weapons

 * Steam Blade - Standard sidearm weapon. Can combine with the Nebula Steam Gun to form the Nebula Steam Rifle. Shares with Remocon Bro's.

Portrayal
Rai is portrayed by Yuudai Nasuda (奈須田 雄大). As Engine Bro's, his suit actor is Keisuke Azuma (東慶介).

As a teenager, he is portrayed by Taiga Watanabe (渡辺 虎雅), who previously played a baby in Kamen Rider Den-O and also the son of suit actor Jun Watanabe.

Etymology
The Kanji that makes up Rai's first name (雷) translates to "thunder". This also explains the meaning behind his prideful and boastful personality: "Loud like thunder".