This article is about a/an ally in Kamen Rider Build.

Akaba (赤羽), born Masaru Ooyama (大山勝 Ōyama Masaru), was one of the boys who hailed from Hokuto, part of the Hokuto Three Crows led by Kazumi Sawatari. He was able to change into the Castle Hard Smash (キャッスルハードスマッシュ Kyassuru Hādo Sumasshu) at will by using the Castle Fullbottle. After being administered the Hazard Trigger, he became Castle Hazard Smash (キャッスルハザードスマッシュ Kyassuru Hāzado Sumasshu).


Becoming Akaba

Masaru Ooyama was a longtime employee on Kazumi Sawatari's farm, alongside Syuuya Aikawa and Syoukichi Mihara. After the Sky Wall Disaster rendered Hokuto's soil infertile, Sawatari used his savings to continue providing for his workers. Eventually, Sawatari volunteered for Faust's experiments, allowing himself to become a weapon of war for the sake of his workers. Out of loyalty, Ooyama and the others volunteered as well, becoming Hard Smashes to aid Sawatari. Ooyama was given the codename "Akaba".

Invasion of Touto

When Hokuto Prime Minister Yoshiko Tajimi declared war on Touto, Akaba was part of the advance guard with Aoba and Kiba. Through his power as a Smash, Akaba launched a premature strike at the city. Disregarding Kiba's warning against disobeying the boss, Akaba called out to their superior who was engaged elsewhere, asking if he was finished. The Weaponry Hero

Akaba and the others later tracked down Sento Kiryu and Ryuga Banjo, and transformed into their Hard Smash forms. After successfully knocking them out of their transformations, Akaba demanded that they hand over their Fullbottles, prompting Aoba and Kiba to argue with him over who should get said Fullbottles. By the time they had pulled themselves together, Sento and Ryuga had absconded.

Later, they found Sento consoling Misora Isurugi as she lamented her role in allowing evil to wage war, and told Sento to hand over his Fullbottles. This time, Sento gave them a challenge with a determination he gained from Misora, using RabbitTank Sparkling Form. However, Akaba, Aoba, and Kiba still quickly gained the upper hand, until Ryuga appeared with the Sclash Driver and transformed into Kamen Rider Cross-Z Charge. Ryuga and Sento easily defeated them, causing them to drop the Phoenix, Robot, Wolf, Smapho, Kuma, and Televi Fullbottles. Kazumi Sawatari arrived shortly after to challenge Sento and Ryuga. Rider Wars Start

Alongside Aoba and Kiba, Akaba interrupted a fight between Grease and Cross-Z Charge, only for Kazumi to state that they had broken the rules of engagement and relinquish the Kuma and Televi Fullbottles to Ryuga as his penance. Soon after, the Hokuto Three Crows received dog tags from Kazumi which had finally accepted from Massugu Ubukata, whom crafted them out of respect; befitting the Castle Hard Smash, Akaba's tag was colored red. Kamen Rider Build: Birth! KumaTelevi!! VS Kamen Rider Grease!


"Boss.. Thank you... for everything!"
―Akaba's final words to Kazumi as he join his fallen friends.[src]

Kazumi left Akaba in a warehouse with the task of protecting the Pandora Box while he, Sento, and Ryuga would be protecting several dummy locations - the intent being flushing out Seito's spy. Before Kazumi left, Akaba proposed to bury Aoba and Kiba's dog tags in the Sky Road passage between Touto and Hokuto. Akaba dutifully kept his eye on the box, with Gentoku Himuro eventually bursting through the door, shocking Akaba, who wondered how he knew the box's true location. Refusing to hand the box over to Gentoku, Akaba transformed into the Castle Hazard Smash while Gentoku transformed into Kamen Rider Rogue. Rogue brutally beats Akaba in his Smash form, even tearing his right Ground Rampart shield from Akaba's Smash body in the process. Rogue was surprised that Akaba kept on fighting, even when he was at his limit. Holding his dog tags, Akaba prayed for Aoba and Kiba to lend him their power as he lunged forward for a final blow, but only for Rogue to send him back flying with a Crack Up Finish Rider punch; Kazumi, with Sento and Ryuga beside him, arrived just in time to watch the fight's end. Dying, Akaba reverted back to his human form and spotted Kazumi. Akaba stood tall and turned to Kazumi. With the last of his strength, Akaba thanked him for everything as he disintegrated in a red smoke, and his dog tags fell to the ground as Kazumi ran toward him, calling him by his birth name. The Idol Awakens


Kazumi crafted a grave for the Three Crows at the Sky Road. However, he held onto their dog tags as he participated in the proxy fight against Seito where, despite being outmatched by Engine Bro's, he was ultimately motivated to overcome his opponent by looking back to his late comrades. The Treacherous Deathmatch Kazumi would continue to fight to ensure the well-being of the late Three Crows' respective families. The Counterattack Hero

New World

New Akaba, New Aoba and New Kiba


Akaba is a delinquent right down to his appearance and personality. He is cruel and careless, not hesitating to attack a civilian population out of boredom. Akaba sees himself as superior to his comrades, barring his boss, who he deeply respect.

Akaba, not unlike the rest of his team, is very petty and immature, even arguing over who would take Sento and Ryuga's Fullbottles after defeating them. He and his team were called idiots by Ryuga within minutes of meeting him.

Despite his negative traits, he and his fellow teammates all wanted to protect their families back in Hokuto. He also feel frustrated since he can't help his Captain much during the war. In the end, he, along with Aoba and Kiba, took Stalk's offer to strengthen them into Hazard Smashes, knowing that it could cost him his life. Kazumi himself commented that Akaba's indeed the most stupid, but also the bravest of the trio.

Aoba's death at the hands of Build using the Hazard Trigger made Akaba, as well as Kiba, initially hostile and violent towards Sento and Ryuga. However, after Kazumi decided to become Sento's ally, so that the group could better protect both Hokuto and Touto from the forces of Seito, Akaba begrudgingly forgave them, eventually forging somewhat of a decent comradery with the Touto Riders.


Castle Hard Smash
KRBu-Castle Hard Smash

Castle Hard Smash

  • Height: 215.3 cm
  • Weight: 150.8 kg

Akaba transforms into the Castle Hard Smash using the Castle Fullbottle.


  • The Castle Hard Smash possesses super strength, capable of delivering powerful punches.
  • The Castle Hard Smash's Ground Rampart (グランドランパート Gurando Ranpāto) shoulder-mounted shields are capable of great defensive utility or ramming ability.
  • The Castle Hard Smash can fire a green energy beam from the Catapulta Cannon (カタプルタキャノン Katapuruta Kyanon) on its forehead.

Appearances: Build Episodes 16-19

Castle Hazard Smash
KRBu-Castle Hazard Smash

Castle Hazard Smash

  • Height: 215.3 cm
  • Weight: 150.8 kg

After being administered with the Hazard Trigger's energy, Akaba can now transform into the Castle Hazard Smash with the Castle Fullbottle.


  • The Castle Hazard Smash possesses super strength, capable of delivering powerful punches.
  • The Castle Hazard Smash's Ground Rampart shoulder-mounted shields are capable of great defensive utility or ramming ability.
  • The Castle Hazard Smash can fire a green energy beam from the Catapulta Cannon on its forehead.
  • The Castle Hazard Smash can launch red energy bolts from its arms.

Appearances: Build Episodes 19-25


Behind the Scenes


Akaba is portrayed by Eishin (栄信). His suit actor as the Castle Hard Smash is Yuji Nakata (中田裕士 Nakata Yuuji).


Akaba's name translates to "red wings", sharing a color theme with Kiba (黄羽 lit. "yellow wings") and Aoba (青羽 lit. "blue wings").


  • Akaba's transformation into the Castle Hard Smash by injecting the Castle Fullbottle into his arm is similar to the Dopants of Kamen Rider W.
  • The shape of his shield resembles the shields in Gundam series.
  • Akaba is the only Smash of the Hokuto Three Crows to have an inorganic motif.



Sento Kiryu - Ryuga Banjo - Kazumi Sawatari - Gentoku Himuro - Evolto - Nariaki Utsumi - Shinobu Katsuragi - Kengo Ino - Killbus - Keiji Uraga
Transformation Gear
Fullbottles - Build Driver - Transteam Gun - (Great) Cross-Z Dragon - Sclashjellies - Sclash Driver - Hazard Trigger - Nebulasteam Gun - Cross-Z Magma Knuckle - Evol-Driver - Evol-Trigger - Grease Blizzard Knuckle - Killbuspider - Grease Perfect Kingdom
Drill Crusher - Hawk Gatlinger - 4Koma Ninpoutou - Kaizoku Hassyar - Fullbottle Buster - Steam Blade - Beat Closer - Twin Breaker - Muscle Glove - Solstall Wings - Giant Scratcher - Space Ride Arm - Spine Knuckle - Long Range Cleaner - Trash Converter - Gold Lio Gauntlet - Multi Deluge Gun
Pandora Box - Halfbodies - Fullbottle Holder - Build Phone
Vortex Finish - Hazard Level - Project Build - Sky Wall - Nebula Gas
nascita: Misora Isurugi - Soichi Isurugi - Sawa Takigawa - Kazumi Sawatari - Gentoku Himuro
Prime Ministers: Taizan Himuro (Touto) - Yoshiko Tajimi (Hokuto) - Masakuni Mido (Seito)
Namba Heavy Industries Ltd.: Juzaburo Namba - Nariaki Utsumi - Guardian
Hell Bro's: Fu Washio - Rai Washio
Aliens: Vernage
Downfall: Keiji Uraga - Simon Marcus
Gentoku Himuro - Evolto - Nariaki Utsumi - Takumi Katsuragi - Shingo Kuwata - Eita Kawai
Touto: Needle Smash - Strong Smash - Burn Smash - Flying Smash - Mirage Smash - Square Smash - Press Smash - Ice Smash - Stretch Smash
Hokuto: Castle Hard Smash - Stag Hard Smash - Owl Hard Smash
Kaiser: Kaisei Mogami (World of Ex-Aid)
Kengo Ino - Ryoka Saiga - Mitsuomi Gobara - Evolto - Killbus