Musume-Kumo is a small, spider-like entity that serves as the central figure in the story of Li'l Spider Girl. Her origins lie in an ancient legend: centuries earlier, a colossal spider terrorized Tokyo until a powerful exorcist sealed it away inside a book. When the book is accidentally unsealed by the impulsive Mizuki Henmi, Musume-Kumo emerges in a diminutive, seemingly harmless form. She has black hair and red eyes, with the appearance of a cute, young girl, which immediately captivates the bookstore owner Shu Suzuri, who becomes deeply infatuated with her. In contrast, Mizuki grows increasingly uneasy, sensing that something dangerous lurks beneath the creature's innocent exterior.

Musume-Kumo's personality is initially portrayed as playful and curious, but she gradually reveals a more ominous and unpredictable nature. Her motivations appear driven by a need to grow stronger or reclaim her former power, as her presence coincides with unsettling events and a growing sense of dread. As the story progresses, she undergoes a transformation, shedding her childlike form and reverting to a monstrous spider kaiju that threatens the city once more. This change marks her development from a cute pet into a genuine threat, forcing the protagonists to confront the consequences of releasing her.

Her notable abilities include the power to grow to enormous size, the capacity to spin webs, and a supernatural influence that seems to affect those around her, particularly Shu. Her key relationships are with Shu, who willingly protects and dotes on her, and Mizuki, who becomes her primary antagonist. The bond between Shu and Musume-Kumo borders on possessive, while Mizuki's wariness eventually proves justified as the spider girl's true nature emerges. Although her time on screen is brief, Musume-Kumo serves as the catalyst for the entire conflict, embodying the tension between appearances and reality that defines the special's horror elements.