Kouhei Nikaido, a private first class in the Imperial Japanese Army’s 7th Division and older twin of Youhei Nikaido, fights alongside his brother at the 1905 Battle of Mukden, aiding wounded soldiers such as Tokushirou Tsurumi and Hajime Tsukishima. Their ruthless loyalty to Tsurumi surfaces during a violent confrontation with Saichi Sugimoto in a soba restaurant, where they attempt to capture the fugitive.

When Youhei dies in a botched assassination against Sugimoto, Kouhei spirals into vengeful obsession, his psyche fracturing as he attributes his own mutilated body—ears torn by a bear and severed by Tsurumi, a shotgun-equipped prosthetic leg replacing one lost to Hijikata Toshizō—to his deceased brother. Subsequent battles further erode his body: a mechanical hand with a hidden chopstick compartment substitutes his right hand, severed by Sugimoto.

Though briefly swayed by Hyakunosuke Ogata’s rebellion, Kouhei returns to the 7th Division, where Tsurumi exploits his instability, providing prosthetics crafted by Narizou Arisaka while manipulating his loyalty. In the Edogai Arc, Kouhei aids Tsurumi’s recruitment of taxidermist Yasaku Edogai, leveraging his own severed ear as a grotesque persuasion tool.

Tsurumi’s fabricated claim of Sugimoto’s death strips Kouhei of purpose, plunging him into catatonic withdrawal. His decline, marked by morphine addiction and self-harm, mirrors the toll of relentless physical and psychological trauma.

Kouhei’s trajectory embodies identity dissolution and war’s dehumanization. Delusions of replacing Sugimoto with an imagined Youhei expose his shattered mind, while dependence on Tsurumi and mechanical augmentations reflects a twisted symbiosis of allegiance, adaptation, and insanity in a fractured postwar world.