Hachirou Kouga is a primary character and the male lead in the anime Basilisk: The Ouka Ninja Scrolls. He is the son of Gennosuke Kouga and Oboro Iga, the two central figures from the original Basilisk: The Kouga Ninja Scrolls story. Together with his twin sister, Hibiki Iga, he was born in the aftermath of the tragic conflict that defined his parents' generation. Because of his lineage, he serves as the leader of the Five Treasures of Kouga, a group of five elite ninja from his father's clan.

Personality-wise, Hachirou is defined by a strong sense of protectiveness, especially toward his sister Hibiki. He carries the weight of his parents' legacy, which includes the knowledge that love between the Kouga and Iga clans has historically led to sacrifice and death. This knowledge deeply affects him, as he initially decides to distance himself from Hibiki in an attempt to avoid repeating the tragic fate of his parents, believing that his absence might protect them both. However, this decision is met with resistance from Hibiki, and his underlying devotion to her remains a constant driving force. He possesses a fierce and vengeful side, particularly when his loved ones are harmed. Following a brutal attack on his village by the Joujinshu clan, where Hibiki is dismembered and turned into a puppet, Hachirou dedicates himself to retrieving her body parts and seeking revenge, showcasing a determined and relentless resolve.

Hachirou's primary motivation is the protection of and reunion with his sister, Hibiki. While the elders and members of their combined clans desire the twins to produce an heir to unite the Kouga and Iga bloodlines, Hachirou's personal journey becomes one of survival and recovery after the Joujinshu's assault. His role in the story is that of the avenging hero, thrust into a violent conspiracy that goes beyond the traditional rivalry of his clans. He leads his fellow Kouga ninja on a mission, which becomes a quest to save Hibiki and confront the supernatural threat posed by the Joujinshu and their leader, Joujin, who seeks to harness the twins' power for a resurrection ritual.

His most significant relationship is, without question, with his twin sister Hibiki. They are seen as a pair destined to be together, a notion complicated by their status as siblings, which is a recurring and controversial theme within the narrative as their clans push for a union between them. Hachirou is the older brother and feels a profound responsibility for Hibiki's safety. He also shares bonds with the other members of the Five Treasures of Kouga, such as Namenba, who acts as a guardian figure and reveals the plan for the twins to have children.

Throughout the story, Hachirou undergoes significant development. He begins as someone who tries to escape his destiny by leaving the village, but the violent intervention of the Joujinshu forces him to confront his fate head-on. He spends six years in self-imposed exile, training to become powerful enough to take revenge on those who harmed his sister. This marks a transformation from a reluctant heir to a decisive and powerful warrior willing to use any means necessary to reclaim what he has lost, even as the story explores themes of sacrifice and taboo.

Notable abilities are central to Hachirou's character. He inherited a powerful dojutsu, or eye magic, from his father, Gennosuke. This technique, often referred to as the Kouga Eye technique, has the ability to turn an opponent's malicious intent back upon themselves, forcing them to destroy themselves. As the story progresses, it is revealed that Hachirou and Hibiki are the vessels of a mystical force known as "Ouka," which manifests as a golden energy when they combine their powers. This combined technique is incredibly destructive and can even shield others, but using it is a dangerous forbidden technique that can cost them their lives. The ultimate goal of the main antagonist is to steal the Ouka power from the twins for his own dark purposes.