Asagi is a half-demon child who appears as a central character in the fourth animated film of the series. She resides on Horai Island, a mystical land that materializes only once every fifty years. As one of a small group of half-demon children living on the island, Asagi is notably the eldest among them. In her early preteen years, she is small and slender but stands taller than the other children. She has distinctive blue hair tied in a ponytail, golden eyes, and the pointed ears characteristic of a half-demon. Her attire consists of a simple brownish-gold dress tied at the waist with a rope, and she wears a beaded necklace featuring violet triangular pendants. Like her companions, she bears the Mark of the Four War Gods on her back, a scar that binds her to the island and marks her as intended fodder for the demonic forces that control it.

Asagi possesses a personality defined by an extraordinary sense of responsibility and a strong will. Forced to mature quickly due to her dire circumstances, she acts with consistent bravery in an effort to serve as a role model for the younger half-demon children. She carries an immense amount of stress and emotional weight, which she hides behind a facade of acceptance and determination. Her primary motivation is the protection and survival of the other children, whom she considers her family. This maternal instinct is so powerful that she fully accepts her own tragic fate, believing that her sacrifice is the only way to buy the others a little more time to live. This conviction makes her notably stubborn; she initially refuses to consider alternative solutions, insisting that her plan is the only path forward. When she feels it is necessary for the greater good of the group, she is capable of showing tough love, such as slapping a younger child for returning to the island or scolding another for cowardice, actions that stem from her deep care rather than cruelty.

Within the story, Asagi functions as the de facto leader and guardian of the half-demon children. Her role is to maintain order, provide comfort, and, most critically, to offer herself as a sacrifice to the Cauldron of Resonance. The Cauldron is a device used by the Four War Gods to slowly regain their power, and Asagi has resigned herself to being consumed by it. When Inuyasha and his companions arrive on the island, she initially rejects their help, fearing it will only lead to more death. Her determination to follow through with her sacrifice serves as a major obstacle and emotional core of the plot, as the protagonists must convince her to hope for another way. Her key relationships are primarily with the other half-demon children, especially the youngest, Ai, whom she sends away from the island to save her from their fate. She also forms a significant bond with the priestess Lady Kanade, whose spirit appears to offer guidance and strength.

Throughout the film, Asagi undergoes a crucial development. She begins as a child resigned to death, having accepted a hopeless future for the sake of her younger companions. Her encounter with Inuyasha and his group, particularly witnessing their refusal to abandon them to their fate, slowly challenges her fatalistic worldview. The turning point occurs inside the Cauldron, where she receives the blessing of Lady Kanade and actively works with Inuyasha to escape and destroy the device. By the end of the story, she is freed from the Mark of the Four War Gods and from the obligation of sacrifice. She evolves from a martyr accepting a tragic end to a survivor who can look toward a future. In the film's conclusion, she chooses to leave the island with the remaining children to explore the outside world, demonstrating a newfound hope and the will to live freely. Her notable abilities are not combat-oriented; her strength lies in her resilience, leadership, and the power of her spirit, which is exemplified by the Song of Parting she sings, a melody that represents both her sorrow and her dedication to those she protects.