Akaba

Akaba is a member of the Hokuto Three Crows and a supporting antagonist in Kamen Rider Build.

Past
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".

Hokuto-Touto War
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.