Hisui Kisaragi is an eighteen-year-old senior high school student at Ouran Academy, where he is also an official member of the Tea Ceremony club. To the outside world, he appears as a reserved and inconspicuous young man who runs an antiques shop, a family business that has long had dealings with the prominent Misato family. This quiet exterior, however, conceals his true nature as the young heir of the Kisaragi clan, a family that has secretly protected Tokyo by hunting demons since the days of the Tokugawa shogunate. From childhood, Hisui was trained by his grandfather in this sacred duty, studying not only ninjutsu but also Shinto rituals and netherworld incantations to battle creatures known as Oni. His primary weapons are kunai, or throwing blades, which he wields with precision.

Hisui's personality is defined by a deep-rooted sense of tradition, duty, and self-restraint. He is very traditional, brooding, and slightly self-righteous, keeping his thoughts and emotions largely to himself. This reserved and introverted nature often puts him at odds with Kyouichi Houraiji, whose loud, brash, and aggressive manner clashes with Hisui's conduct. He is not afraid to voice his disapproval, frequently calling Kyouichi a simpleton and dismissing him as a troublemaker. Despite his calm and controlled demeanor, Hisui is fiercely devoted to his beliefs and the responsibilities his clan has carried for generations.

His motivations are deeply personal and rooted in his past. He has been close to Aoi Misato since they met as small children, drawn to her warmth, sincerity, and sympathy. This childhood connection blossomed into a psychologically deep and loyal affection, though he almost never speaks his true feelings aloud. He received a charm bracelet from Aoi when they were younger, which he keeps in a pouch around his neck, and he made a solemn promise to always protect her, addressing her with the honorific Aoi-sama. This devotion forms the core of his character, creating a powerful internal conflict when he discovers that Aoi is the vessel of the Bodhisattva Eye, a power that the Kisaragi clan had sworn to destroy. Torn between his duty, his honor, and his affection for her, he ultimately finds himself unable to fulfill his family's oath.

In the story, Hisui serves as the knowledgeable guide and mentor to the other teenagers who have awakened to supernatural powers following a disruption of the Ryumyaku, the flow of arcane energy through the city. Unlike the others, he is the only one who fully understands the true dangers Tokyo faces, and they turn to him for guidance and knowledge about demons and the hidden war being waged in the city's shadows. He fights alongside the group but also acts as a bridge to the ancient world of demon hunting and spiritual warfare. His role also encompasses a cosmic significance, as he is revealed to be the Vessel of Genbu, the Black Tortoise, one of the four deities destined to protect the Golden Dragon. His Star of Fate falls under this celestial guardian, marking him as an essential part of the larger destiny at play.

Throughout the series, Hisui's primary development revolves around the struggle between his rigid upbringing and his personal emotions. His loyalty to Aoi and his hidden feelings for her are tested to their limits as he grapples with the knowledge that his clan's mission is to eliminate the very power she now possesses. This internal battle forces him to confront the meaning of his duty and what he is willing to sacrifice for honor versus what he will protect for love. By the end of the second season, he chooses to follow his own path, joining Kyouichi to search for their ally Tatsuma, demonstrating a shift from solitary duty to shared purpose. In combat, his abilities are rooted in his clan's techniques; he is a highly skilled ninja and demon hunter who employs kunai for both attack and defense, supplemented by his knowledge of Shinto and dark incantations, making him a formidable and strategic fighter against supernatural threats.