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This article is about a/an villain in Kamen Rider Build.
―Transformation phrase[src]

Rai Washio (鷲尾 雷 Washio Rai) 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 Enjin Burosu).

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. The Phantom of the West

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. A Man Called Rogue


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. Rai seems to dislike Gentoku, calling him Seito's Kamen Rider in disgust.

Nevertheless, both Fu and Rai are cruel and ruthless, and likes to torture their defeated opponents.


Engine Bro's
KRBu-Engine Bros

Engine Bro's

"Gear Engine! Funky! (Rock music, engine revving) Engine Running Gear!"
―Transformation announcement[src]


  • Height: 203.0 cm
  • Weight: 103.5 kg

Ability Perimeters:

  • 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.

One half of the Hell Bro's, Engine Bro's (エンジンブロス Enjin Burosu) 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.

Against Kamen Rider Grease, Rai and his brother were easily able to overcome the Hokuto Rider and two of the Hokuto Three Crows in their Smash Hazard forms. In their battle with Kamen Rider Build Rabbit Tank Sparkling and Cross-Z Charge they had a slight advantage and were able to resist their Sparkling Finish and Scrap Break without much difficulty.

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 Fankī Doraibu): 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 Fankī Atakku) 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 Fankī Shotto): 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, ROGUE




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

Behind the Scenes


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

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


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".