Shouko Mataki is Eiri Kurahashi's close friend and confidant, often interacting with him at his family's antique shop where he works. She displays a neurotic and insecure personality, reacting with visible jealousy to news about Eiri's romantic life. Her concern deepens as Eiri grows increasingly withdrawn and physically afflicted by supernatural experiences, leading her to question the source of his suffering.

She voices distress over Eiri's worsening condition, directly challenging the morality of anyone causing such harm under the guise of love. When Eiri's physical wounds appear, she confronts the contradiction between professed affection and the reality of his torment. Driven by protective instincts, she seeks explanations and solutions, coordinating with other women in Eiri's life who observe his self-destructive path.

During the narrative's climax, Shouko falls comatose after an encounter with the antique shop's supernatural forces, an event her aunt, local physician Hatsumi Mataki, finds medically unexplainable. After Eiri confronts the illusory Cossette and creates a new blood-portrait, Shouko awakens. Her revival coincides with the liberation of the real Cossette's soul, which subtly blends with her consciousness. This spiritual integration colors her perspective and dialogue upon waking, revealing an intuitive grasp of Eiri's artistic future that suggests a lingering connection to Cossette's essence. Her final words convey both her original care for Eiri and a new, transcendent awareness born from this merging of spirits.