Icon-build.png This article is about a/an ally in Kamen Rider Build.

Akaba (赤羽), born Masaru Ooyama (大山勝 Ōyama Masaru), is one of the boys who hailed from Hokuto, part of the Hokuto Sanbagarasu 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 Sanbagarasu 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]

The death of Akaba.

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 Sanbagarasu 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 Sanbagarasu' respective families. The Counterattack Hero

New World

New Akaba, New Aoba and New Kiba.jpg

Akaba, along with his friends, were among the few people who has a counterpart on the World without Sky Wall, and was seen as one of the delivery men who frequents Nascita. On one occasion, they dragged their boss to see the cute girl from the coffee shop they frequent in, which ended up with their boss passing-out after seeing Misora.The Tomorrow Build Will Create When Killbus reactivated Pandora's Box, everyone who were directly exposed to Nebula Gas in the original World of Build regained their memories from that world, including Akaba. Although he didn't participate in the fight against Killbus, they were mentioned by Kazumi in the meeting.Build NEW WORLD: Kamen Rider Cross-Z

Akaba Grease.png

Because of their memories being restored, Misora's attitude towards Kazumi went back to the way they were in the original world, which worried Akaba and the others. After a misunderstanding between Kazumi and Misora, Akaba and the others did their best to cheer their boss, to the extent that they were willing to join him in his self-searching.I Heard the Idol Otaku is Dating His Fav This led them to an island which was formed from the point of contact between the two worlds that merged. Kiba felt something dangerous from the island at first, but calmed down after finding a hot spring. While they were bathing in it, a geyser suddenly emerged, which soaked the four even further to the water. After being soaked, the four went to a cave to try and warm themselves up, as they were feeling some strange after-effect from the water. They would later recover and find their way back to the Sawatari Farm, only to find it attacked by a foreigner, which Massugu described as a green monster. Confronting the perpetrator, they were confused on to what the attacker wanted to do, and was more confused when it suddenly flew away after it saw them transform. Kazumi then contacted Gentoku, who hinted that he was captured. After the call, they then went to Sento's place to find out more. Given the task to safeguard Misora, Akaba left with Kiba and Aoba, leaving behind Kazumi due to the trauma their boss still have. Arriving just before Down Fall took Misora, they were confronted by Kamen Rider Metal Build and were taken out all at once. Regaining consciousness just after Sento was explaining the power-up item to Kazumi, the Sanbagarasu waited for their boss to leave, before willingly agreed to help Sento. Sento then explained that he will need the three to give-up a majority portion of their Phantom Liquids, the extraction was painful, but they were able to gather enough for four and complete Grease Perfect Kingdom. After Kazumi successfully defeats Kamen Rider Phantom Build, Akaba and the others revealed to Kazumi that they had to play along Sento's plan to elevate Kazumi's Hazard Level by believing them to be dead, from which Kazumi calls Sento the Devil Scientist again.Build NEW WORLD: Kamen Rider Grease


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 felt 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 is 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

Castle Hard Smash

  • Height: 215.3 cm
  • Weight: 150.8 kg

Akaba transforms into the Castle Hard Smash (キャッスルハードスマッシュ Kyassuru Hādo Sumasshu) 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

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 (キャッスルハザードスマッシュ Kyassuru Hāzado Sumasshu) 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

Castle Lost Smash

KRBu-Castle Lost Smash.png

  • Height: 215.3 cm
  • Weight: 150.8 kg

The New World counterpart of Akaba transforms into the Castle Lost Smash (キャッスルロストスマッシュ Kyassuru Rosuto Sumasshu) using the Castle Lost Fullbottle.

This form is exclusive to Build NEW WORLD: Kamen Rider Grease


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").

Concept Art

Castle Smash was designed by Tamotsu Shinohara (篠原 保 Shinohara Tamotsu).


  • 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 Sanbagarasu to have an inorganic motif.



External Link

  • Akaba in Kamen Rider Official Website
Sento Kiryu (Takumi Katsuragi) - Ryuga Banjo - Kazumi Sawatari - Gentoku Himuro - Evolto - Nariaki Utsumi - Shinobu Katsuragi
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nascita: Misora Isurugi - Soichi Isurugi - Sawa Takigawa - Kazumi Sawatari - Gentoku Himuro
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