Sanpei acts as one of Tenkai's subordinates, initially assigned to train Jiro in ninjutsu at the Ezo temple after Jiro flees his village. He serves as Jiro's mentor during these formative shinobi years. Later, he is revealed to be a covert operative for the Satsuma domain, opposing Tenkai's master, the Tokugawa Shogunate. As a mole within Tenkai's organization, Sanpei leaks information and manipulates events to undermine the Shogunate's secret operations. During the confrontation in America, his prior association with Jiro's father, Tarouza, is uncovered, linking him to Jiro's past and revealing a deeper personal stake.

Despite maintaining his facade of loyalty to Tenkai, Sanpei's true allegiance leads him to indirectly protect Jiro throughout the latter's journey. This culminates in his participation with the pro-imperial faction in the Boshin War, aligning with figures like Saigō Takamori to overthrow the Shogunate. He is present during the Imperial forces' assault on Hakodate, continuing his role in the broader conflict. Sanpei survives beyond the Tokugawa regime's fall, positioning himself within the new Meiji power structure alongside Saigō Takamori.