Born into a prominent family through adoption, he was groomed to inherit its legacy, yet endured relentless contempt from his adoptive mother, deepening his emotional isolation. A maple leaf-shaped burn scar etched across his back served as both a physical reminder of trauma and a symbol of fractured identity. Childhood bonds with servant Yaichi and neighboring heir Yagi provided fleeting solace before a staged kidnapping orchestrated by his mother upended his life. Bandits under Jin exposed her plot to eliminate him, deceitfully framing Yaichi as a traitor. Shattered by betrayal, he abandoned his name, adopting "Yaichi" as a hollow tribute to the servant he believed had forsaken him.

Reborn as "Sei the Drifter" within Jin’s gang, he embraced merciless raids targeting civilians, including women and children, his morality eroded by vengeance. Later, he betrayed the gang, withholding warnings of pursuing authorities to abscond with their riches—an act of ambiguous intent. Shifting tactics, he formed a group to kidnap corrupt nobles’ heirs, masking self-enrichment as societal reckoning. This duality defined his leadership until Jin resurfaced, exposing Yaichi’s innocence. The revelation shattered his constructed persona, driving him to kill Jin in the anime’s climax, while the manga spared the bandit.

Allies like Masanosuke steadied him during his collapse. At Yaichi’s grave, he confronted his guilt before departing Edo with companions, tentatively embracing collective redemption.