Gennosuke Kouga is the heir to the Kouga clan, assuming leadership after the death of his grandfather, Danjo. His background is steeped in a centuries-old, bitter blood feud with the rival Iga clan, a conflict he is determined to end. From a young age, he was placed under the care of his uncle, Hyoma Muroga, who trained him not only in swordsmanship but also in the use of his inherited ocular ability. This training also involved learning to fight while blindfolded, a discipline that would later prove crucial.

Gennosuke is characterized as an idealist, a scholar, and a man of peace. He firmly believes in looking towards the future instead of being dominated by the hatreds of the past and views the pursuit of vengeance as pointless and self-destructive. Despite his fierce appearance, he possesses a calm, gentle, and sincere personality. He is deeply in love with Oboro, the granddaughter of the Iga clan's leader, and this relationship is the cornerstone of his desire to reconcile the two warring factions. The engagement was arranged to bring peace but, unlike many such arrangements, Gennosuke and Oboro willingly and deeply love one another. Even as the body count rises around him, his feelings for her remain, causing him great torment as he struggles to balance his personal heart with his duty as his clan's leader.

His primary motivation is the absolute rejection of the ancient cycle of violence between the Kouga and Iga. When the long-standing Hattori truce is dissolved by the shogun, forcing the clans to send their ten best warriors to fight to the death, Gennosuke refuses to participate in the slaughter. Instead of waging war, he leads his clansmen on an expedition to Sunpu Castle to directly plead with the shogun and his advisor to reinstate the treaty before more bloodshed occurs. He seeks to understand the political machinations forcing them to fight, hoping to find a peaceful resolution and secure a future where he can be openly married to Oboro. His actions are driven by an enterprising desire to resolve a thousand years of resentment.

In the story, Gennosuke serves as the reluctant leader of the Kouga clan. As the conflict is forced upon them, his role evolves from a peace-seeking heir to a protector of his people. He adheres to his ideals even as his comrades are killed, but he is also more than capable of deadly force when he or his clansmen are cornered. His journey is one of tragic inevitability; he tries to swim against the current of a political conspiracy designed to destroy his kind. In the climax, having witnessed the annihilation of both clans' warriors, he faces Oboro. Unable to reconcile his love with the duty to kill her, he resigns himself to death at her hands.

His key relationships are central to his character. His love for Oboro is the most significant, symbolizing hope for peace. He carries a flute in the anime which he often plays for her, representing their shared dream. His relationship with his uncle and teacher, Hyoma Muroga, is also crucial. Hyoma, who shares the same Doujutsu ability, taught Gennosuke not only how to use his power but also the humanist beliefs that allow him to see past the clan hatred. Despite his peaceful nature, he respects his fellow Kouga warriors, though his authority is sometimes challenged in favor of more aggressive actions.

Gennosuke undergoes a devastating development arc. He begins as an optimistic visionary, confident that love and reason can overcome ancient hatred. As the conflict grinds on and his friends and family are killed, he is filled with torment. He does not become a vengeful killer but rather becomes increasingly resigned to his tragic fate. His final act is one of compassionate defiance; he cannot kill Oboro, so he allows her to be the victor. When she commits suicide instead, he records her as the survivor, denying the shogunate a true winner, before taking his own life to join her.

Gennosuke possesses several notable abilities. His most feared power is the Doujutsu, a hypnosis technique passed down his bloodline. This ability reverses the homicidal intent of anyone who meets his gaze, compelling them to kill themselves instead of harming him. Unlike his uncle, Gennosuke can activate and deactivate this power at will. He is also an extremely skilled swordsman, capable of matching and defeating opponents like Yakushiji Tenzen, who has centuries of experience, even when Gennosuke is temporarily blinded. Due to his rigorous childhood training with his uncle, he possesses highly refined senses, allowing him to function and fight effectively even without his eyesight. This proves essential when the Iga use the Seven Days of Darkness ointment to temporarily blind him and seal his Doujutsu.