Gennosuke Kouga is the heir to the Kouga Manjidani clan and its leader following the death of his grandfather, Danjou. He is an idealist and a scholar who firmly believes in looking toward the future rather than being controlled by the hatreds of the past. He views the pursuit of any grudge as ultimately pointless and self-destructive, a philosophy that guides nearly all of his actions. His primary motivation throughout the story is to break the centuries-old cycle of violence between the Kouga and Iga clans, a goal he initially seeks to achieve by marrying Oboro, the granddaughter of the Iga clan leader.

His personality is defined by a calm, sincere, and gentle demeanor. Despite his fierce appearance and deadly capabilities, Gennosuke is a pacifist at heart. Even after the sacred Hattori truce between the clans is formally absolved and a deadly competition is imposed, his first instinct is not to fight but to lead his people to Sunpu Castle. He intends to directly plead with the former shogun to understand the rationale for the decree and reinstate the treaty before any more blood is shed. However, this commitment to peace does not signify weakness. When pushed into a corner, or when his clansmen are threatened, Gennosuke is more than capable of lethal action, showing a resolute willingness to kill enemies who attack him or his allies.

Gennosuke's role in the story is that of the tragic hero and the leader of the Kouga contingent. He is thrust into leadership after his grandfather's death and must navigate a brutal conflict he never wanted. His deepest relationship is with Oboro, the Iga princess. Their love is sincere and deep, representing a shared hope for reconciliation. He carries a flute that he often plays for her, a symbol of their dream to unite the clans. This relationship becomes his greatest source of torment, as he struggles to reconcile his personal feelings for her with the duty he owes to his clan as their leader while the body count rises around them. His other key relationship is with his uncle and mentor, Hyouma Muroga, who trained him not only in the use of his Dojutsu but also in how to fight using only his other senses while blindfolded.

Gennosuke undergoes significant development as he is forced to abandon his idealistic hopes for a simple, peaceful resolution. He begins as an engaged young man envisioning a future without war, but he transforms into a tormented leader who must guide his people through a gauntlet of assassins. He ultimately accepts that he cannot avoid facing his beloved Oboro. In the final confrontation, resigned to his fate, he resolves to let her kill him. However, Oboro kills herself instead, forcing Gennosuke to live with the pain of her sacrifice. He records her as the final survivor of the contest before taking his own life to be with her, an act that speaks to his profound sense of loss and his rejection of the meaningless victory the conflict demanded.

Gennosuke's most notable ability is a powerful inherited technique known as Dojutsu. This is a form of hypnosis that reverses the homicidal intent of anyone who directs hostility toward him. In practice, if an enemy looks at Gennosuke with the will to kill him, the ability compels that person to commit suicide instead. Unlike his uncle's version of the technique, Gennosuke can activate his at will, and when he does, his corneas turn crimson red while his pupils shine a brilliant gold. Because of this power, he is a target of immense fear, leading the Iga to temporarily blind him with a special ointment to level the playing field. Thanks to his grueling training with Hyouma, his other senses are so enhanced that he can function and fight effectively even when completely unable to see. Apart from his dojutsu, Gennosuke is also an extremely skilled swordsman. His style is more akin to a samurai than a ninja, and his ability with a blade is so great that he is able to match and defeat Tenzen Yakushiji, an enemy with two centuries of combat experience, even while Gennosuke himself was blinded and wounded.