Kotaro Fuma, whose true name is Isame Fuuma, is a figure from the Fuma clan, a group of masterless ninja bound by their own traditions. He and his older twin brother, Shirasu Kinjo, are the joint tenth heads of their clan, secretly sworn to a centuries-old duty of resurrecting the ancient serpent demon known as the Orochi. Ten years before the events of the main story, the brothers faked the massacre of their own clan to go into hiding. While Shirasu infiltrated the Kumō household to keep watch over the vessel of the Orochi, Kotaro took on the name Kotaro Fuma and disappeared into the high-security prison Gokumonjo.

Within the prison, Kotaro is a quietly menacing presence, often seen wearing a fox mask. He is a cunning and patient schemer, establishing a power base by smuggling opium to the inmates and turning them into an army of addicts loyal to him. His central motivation is his unwavering duty to his clan and the planned resurrection of the Orochi, a task he executes with cold calculation. Despite working toward the same goals as his brother, he did not participate in the murder of the Kumo brothers parents, an act he attributes entirely to Shirasu.

As an antagonist, Kotaro operates on a separate track from the main conflict, his control over the prison population making him a formidable obstacle for the Kumo family and the Orochi Extermination Squad. His role highlights the depth of the Fuma clans intricate conspiracy. His greatest abilities lie in his intellect, his talent for manipulation, and his strategic patience, traits that allow him to build a criminal enterprise within the heart of the prison system.

After the defeat of the Orochi, Kotaro is cornered by the Kumo siblings. Rather than submit, he chooses to leap from a cliff to his apparent death. Later developments in the story reveal he survived the fall, returning to the restored Fuma clan. He carries with him a lingering and complicated sense of fondness for the brothers he once fought against.