Sanpei, a subordinate of Tenkai, trains the protagonist in ninjutsu during their youth while covertly serving as a mole for the pro-imperial Satsuma-Chōshū alliance against the Tokugawa Shogunate. Initially perceived as loyal to Tenkai’s faction, his true role involves sabotaging Shogunate interests from within. During a pivotal clash, Tenkai exposes Sanpei’s past alliance with Tarōza, the protagonist’s biological father, revealing deeper ties to the protagonist’s lineage. Sanpei’s clandestine actions indirectly support the protagonist’s resistance against Tenkai’s schemes. He participates in the decisive Battle of Hakodate, where imperial forces overwhelm Shogunate loyalists. The narrative implies his survival and continued allegiance to the imperial cause post-conflict, though his subsequent endeavors remain unexplored.