Aoba

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

Past
Syuuya Aikawa was originally a worker at Kazumi Sawatari's farm along with Masaru Ooyama 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.

During a fight between Aoba and Kiba, Sento and Ryuga, Sento managed to access his Build RabbitTank Hazard Form and, succumbing to the Hazard Trigger's side effects, Build went into a brutal rampage that claimed the life of Aoba. After knocking Sento out of his transformation, Kazumi went over to a dying Aoba, who handed Kazumi his dog tag before dying and disintegrating into blue smoke.