Akaba is a member of the Hokuto Three Crows and a supporting protagonist in Kamen Rider Build. He originally took part in the Hokuto invasion of Touto but later defected to Team Build along with his boss Kazumi Sawatari after Seito occupied Hokuto.
Masaru Ooyama was originally a worker at Kazumi Sawatari's farm along with Syuuya Aikawa and Syoukichi Mihara. When the Skywall Disaster made Kazumi's land infertile, Kazumi used his savings to continue providing for the three and their families. Eventually, after the Hokuto government made a pact with Faust, Kazumi volunteered himself for the Faust organization's experiments so he could continue to support his workers, becoming Kamen Rider Grease and a soldier of the Hokuto government.
Out of loyalty, Masaru, Syuuya and Syoukichi joined the Hokuto military as well and volunteered themselves to be converted into Hard Smash to aid Sawatari as the Hokuto Three Crows. Becoming the Castle Hard Smash, Masaru was code-named "Akaba".
After Prime Minister Yoshiko Tajimi declared war on Touto, Akaba and the other Crows were sent in to spearhead the invasion alongside Kazumi/Kamen Rider Grease.
However, after Hokuto sent in all their troops to invade Touto, Seito took the opportunity to invade and occupy Hokuto, easily conquering the country. After Kiba was killed by Seito's Remocon and Engine Bro's, Akaba and Kazumi took refuge with the nascita crew.
Akaba later participated in a scheme to lure out a spy in the Touto government, guarding the Pandora Box while Sento, Ryuga and Kazumi guarded other fake locations. However, Gentoku Himuro managed to find the real location and confronted Akaba to take the Pandora Box from him. Transforming into the Castle Hazard Smash, Akaba fought against Gentoku/Kamen Rider Rogue but was unable to overcome the Rider's fierce fighting style and was defeated. As the fight ended, Kazumi, Sento and Ryuga arrived only to find Akaba dying. As he disintegrated, Akaba thanked Kazumi for all he had done for him before fading into red smoke.
Return as a clone
A clone of Akaba along with clones of the other Hokuto Three Crows were later created by Evolt from his DNA and sent after the four Kamen Riders as they stormed the Pandora Tower. Grease opted to stay behind to fight them, telling his fellow Riders to go on ahead without him. After transforming into Grease Blizzard, Kazumi managed to defeat and destroy all three clones though he died soon after as a side affect of transforming with Blizzard Knuckle.
In the new world created by merging the World of Build into the main Kamen Rider world, Akaba as well as Aoba and Kiba was still a worker at Kazumi's farm. He was seen eating at the nascita cafe with Kazumi and his coworkers and had to catch his boss when he fainted after falling in love with Misora at first sight.
He later assisted Kazumi in his fight against the Downfall terrorist organization.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga