Yaichi, originally named Seinoshin, was adopted into a prominent family. Rejected by his foster mother after she bore a biological son, she orchestrated his kidnapping by bandits led by Jin, intending his death. Jin revealed the plot and offered a choice: return home to certain death or join the bandits. Traumatized by Jin's lie that his trusted servant Yaichi had betrayed him, he chose the bandits, becoming "Sei the Drifter" within the Bakuro gang. During this time, he committed violent acts, including targeting women and children during robberies, earning a reputation for brutality. A distinctive maple leaf-shaped burn scar on his back marked his past.

Years later, he betrayed the Bakuro gang by withholding warnings of a police raid, leading to their capture while he escaped. Relocating to Edo, he shed his former identities and adopted the name "Yaichi" in honor of his deceased servant. There, he freed Otake from brothel slavery and formed the Five Leaves. As their leader, he coordinated kidnapping operations targeting wealthy families, demonstrating charisma and strategic acumen. He resided in a brothel, protecting its workers while maintaining emotional distance from his past. His philosophy emphasized living solely in the present.

His carefully constructed persona unraveled when Jin reappeared, revealing the original Yaichi had not betrayed him and likely died opposing the kidnapping. This triggered a profound emotional collapse, culminating in Yaichi breaking down at his namesake's gravesite. In the anime adaptation, this confrontation ended with Yaichi killing Jin. Following this, he attempted to disband the Five Leaves but was persuaded to continue by Masanosuke Akitsu's loyalty. The manga depicted a divergent resolution: arrested by his childhood acquaintance Yagi, Yaichi refused to implicate his comrades despite torture. This act solidified the Five Leaves' loyalty, leading them to leave Edo together for a fresh start.

Throughout the series, Yaichi's relationships with the Five Leaves members evolved from transactional ties to mutual dependence. His initial aloofness and manipulative tendencies—such as recruiting Masanosuke for amusement—gradually gave way to vulnerability, like accepting Masanosuke's support during his breakdown and relying on the group during crises, noted by comrades as uncharacteristic. His internal conflict centered on reconciling his violent history with his desire for belonging, marked by isolation, eventual trust in his found family, and the burden of redemption.