Kaguro, originally a human named Genichiro Kuroda, is a primary antagonist and a formidable ayakashi affiliated with the Kokuburo organization. His past as a living person was defined by an obsessive drive to become stronger as a swordsman. Despite being a top student at his dojo, he was unable to defeat his friend and rival, Sakai. Misinterpreting advice that his strength came from a lack of hesitation, Kuroda began challenging and killing numerous swordsmen in his quest for power. After being expelled from his dojo, he encountered a Kekkaishi who sensed the darkness within him and declared him to be no longer human. Terrified by the sight of a Zekkai, he fled to a riverbank where ayakashi invited him to cross over to their world. When Sakai appeared in a final attempt to save his soul, their duel ended in tragedy as Sakai hesitated, and Kuroda killed him, completing his transformation into an ayakashi and gaining entrance to their realm.

As a member of Kokuburo, Kaguro is one of the most powerful ayakashi within the organization, holding a great deal of influence even though he does not attend the leaders' meetings. His power is such that he is one of the few able to resist the control worms used by the leader Byaku, and other demons are consistently wary of him. He is often sent to the sacred land of Karasumori, wearing a human skin disguise to observe battles against the Kekkaishi. He prefers an older model of the disguise, considering the limits it places on his power to be a welcome challenge.

Personality-wise, Kaguro is a cunning, calculating, and deeply sadistic individual. Rather than engaging in indiscriminate slaughter, he typically hangs back during conflicts, observing his opponents until he finds a single one who captures his interest. Once a target is chosen, he relentlessly pursues and provokes them with psychological warfare and mind games, aiming to enrage and unbalance them before a final confrontation. He enjoys manipulating other demons and feels no loyalty toward allies, claiming to have none and cutting down his own subordinates without a second thought, often for what he perceives as a lack of aesthetic or honor. Despite his remorseless nature, Kaguro possesses a twisted sense of personal honor; he despises the taking of hostages and prefers direct combat, once killing his own men for using a hostage against the Kekkaishi because it disgusted him. His motivations are rooted in a singular, consuming desire for power and the thrill of battle. He believes himself to be an outsider rejected by the world and tries to convince his enemies to abandon their humanity as he did, seeking to find a worthy opponent who can understand and challenge his philosophy of strength.

In the story, Kaguro serves as a major early threat to the protagonists, acting as a direct rival to Yoshimori Sumimura. He leads an assassination squad for Kokuburo and is sent to negotiate with or eliminate the Kekkaishi protecting Karasumori. He brutally wounds the Kekkaishi Gen Shishio, and later taunts and kills him when Gen rejects his offers to join the ayakashi. This event becomes a pivotal moment, as Gen was a mentor-like figure to Yoshimori. During his confrontations with the young Kekkaishi, Kaguro deliberately toys with him, pushing him to discover the power of Zekkai. He recognizes the technique from his past, explaining that it is born from feelings of hate, grief, and rejection. He acts as a catalyst for Yoshimori's growth, forcing the protagonist to reevaluate his convictions and the meaning of true strength.

Key relationships define Kaguro's role. His primary relationship is with Yoshimori, whom he sees as a promising opponent whose spirit and potential mirror his own struggles. He attempts to mold Yoshimori into a worthy adversary by trying to make him abandon his humanity. His past relationship with Sakai is critically important; Sakai was a friend who believed in his potential and ultimately died trying to save him, an event that served as the final push for Kaguro to become an ayakashi. His dynamic with Gen Shishio is also significant, as he sees a kindred violent spirit in Gen, but ultimately kills him for Gen's refusal to fully embrace a demonic path.

Throughout his development, Kaguro reveals more of his human past and philosophy. He shows no regret for his transformation, instead viewing the world that rejected him as something to be destroyed or dominated. His powers continue to grow in combat; as he fights, the strength of his swords increases, eventually becoming unbreakable against even the Zekkai that initially frightened him as a human. He meets his end at the hands of Yoshimori, where in his final moments he experiences a fleeting sense of satisfaction, realizing his desire to make the act of slaying his enemies last forever.

Notable abilities define Kaguro as a fearsome combatant. He is a superior swordsman who can generate katana-length blades from virtually any part of his body, most commonly his palms. Wounds inflicted by his swords have a unique property: they suppress cell activity, preventing regeneration in both ayakashi and half-ayakashi, leaving wounds open and bleeding far longer than normal. He possesses superior speed, able to move so fast that he can disappear from the sight of Kekkaishi, place objects on their spiritual spears without them noticing, and easily evade rapid attacks. He is a master strategist who learns and adapts during battle, reading his opponent's movements and exploiting weaknesses in techniques like the Kekkai. While wearing his human skin, his power is limited, but he considers this a fun challenge, and his true ayakashi form resembles a mummy with badly burned skin, large red and yellow eyes, and a black robe with red trim.