Genpachi Inukai is one of the eight Hakkenshi (dog warriors), marked by three defining traits: a family name starting with "inu" (dog), a peony-shaped birthmark beneath his left eye, and possession of a spiritual bead inscribed with the Confucian virtue "shin" (信), signifying faith, integrity, or sincerity.

He is the son of a fisherman who trained under a renowned martial artist, honing his combat skills. At the series outset, the Shogunate imprisons him, likely for uncontrollable behavior or his sharp tongue. Authorities offer him freedom and restoration of his former position if he assassinates fellow dog warrior Inuzuka Shino, bearer of the cursed sword Murasame. This mission initially sets Genpachi against Shino.

Genpachi possesses a fierce, confrontational nature, often acting as an enforcer. His fighting style is aggressive, and he readily engages in violent confrontations. Early in the narrative, he duels Shino, unaware of their shared destiny as dog warriors. The intervention of Kanamari Daisuke (disguised as the monk Chudai) resolves this conflict, leading to Genpachi's reconciliation with Shino and his integration into the group.

He shares a pre-existing friendship with fellow warrior Inuta Kobungo, which influences his group interactions. Genpachi is also associated with a supernatural entity named Raiki, referred to as a "lightning demon," though the source material does not elaborate on the exact nature of this connection or any transformative abilities.