Masaki Amakasu is a young male member of the Uesugi Bushi elite squad. He possesses a well-built physique and long dark blue hair tied up, featuring a distinct light blue lock at the front. He wears the squad's black armor, complemented by a yellow cloth at his waist.

His background originates from a small mining town formerly controlled by Oni sympathizers. Approximately ten years before the depicted events, when he was nine, the Uesugi Bushi liberated his hometown. Following this liberation, Masaki joined the Band of Bushi, remaining part of their ranks since.

During the Awaji Island Recapture Operation, Masaki participates in combat against Oni. He works alongside commander Kanetatsu Naoe, assisting with battlefield documentation. Masaki expresses a desire to boast about defeating multiple lesser Oni simultaneously, though Kanetatsu insists on precise record-keeping over vague claims. He reacts with concern when questioned about potential tax underpayment by his household, swiftly denying any wrongdoing.

In combat, Masaki follows tactical instructions, maintaining formation during Oni attacks. He witnesses Kanetatsu's large-scale elimination of Oni and later joins reinforcements aiding protagonists Musashi and Michiru against Oni forces. He observes the Oni's interest in the "obsidian goddess" and the subsequent intervention of high-ranking Uesugi members Kuroko Usami and Tatsuomi Uesugi.

His combat abilities include wielding a standard Kitetsu sword, consistent with other Bushi fighters. His childhood name was Misaomaru, with "Misa" meaning manipulation, chastity, fidelity, or faith, and "Maru" signifying roundness, completeness, or purity.