Kojiro stands as the central protagonist throughout the Fuma no Kojirou saga. A member of the historical Fuma ninja clan, he displays exceptional combat skills during conflicts. After the Sacred Sword War, he rejects the cycle of violence and destroys all ten Sacred Swords before vanishing for three months.

He reappears during the New Fuma Clan rebellion, rejoining the fight to protect his clan alongside allies Ryouma, Shoryu, Kirikaze, and Ryuho. This rebellion pits them against former clansmen Muma, Raizou, and the mind-controlling faction leader Shimon. Kojiro faces the harsh reality of battling fellow clansmen to eliminate this internal threat.

His development centers on refusing unnecessary killing, though the rebellion's severity forces difficult choices. A pivotal moment occurs when Kirikaze, dying in Ryouma's arms, writes his killer's name in his own blood. Kojiro's visceral reaction to this loss highlights his profound loyalty to the clan.

In the sequel manga *Fuma no Kojirou: Yagyu Ansatsujou*, Kojiro transitions into a mentor, training the next Fuma generation. This story involves the Yagyu clan, specifically Ranko Yagyu, whose past actions entangled the Fuma in Hakuo Gakuin's conflict against Seishikan. Kojiro allies with mercenary Soushi Date to combat the organization Karma and its Four Sword Kings. Karma revives deceased Fuma members, including Kojiro's former clan brothers, turning them against him in new conflicts.