The Monster Spider in Li'l Spider Girl is a jorogumo, a yokai that typically takes the form of a creature with the upper body of a human female and the lower body of a giant spider. This particular jorogumo is the child of the colossal spider that once terrorized Tokyo centuries ago. An exorcist defeated the mother but could not bring himself to kill the child yokai, so he sealed her inside a book for her own protection. In the present day, the seal is accidentally broken by a young girl named Mizuki Henmi, releasing the jorogumo as a small, cute spider-like entity.

At first, the Monster Spider appears innocent and harmless, quickly endearing herself to the bookstore owner Shu Suzuri, who becomes infatuated with her. She displays childlike curiosity and affection, especially toward Shu, but her true nature gradually surfaces. Mizuki grows suspicious of the creature, sensing an underlying menace. The jorogumo’s motivations revolve around a deep-seated loneliness and a desire for belonging, possibly seeking a mother figure or a place in the human world. As the story progresses, she reveals a darker, more manipulative side. She is capable of taking on a fully human appearance, and in the climax she uses this ability to attempt to kill and replace Mizuki, showing that her innocent facade conceals a predatory yokai instinct.

Key relationships include her bond with Shu, who treats her like a pet or a friend, and her tense dynamic with Mizuki, who represents both the one who freed her and the one who sees through her disguise. The exorcist who sealed her away also serves as a background figure from her past. Her abilities include shape-shifting between a tiny spider form, a human girl form, and a larger jorogumo form with a spider lower body. She can produce and manipulate spider silk, and as a yokai, she possesses supernatural strength and cunning. Her development is a shift from an apparently cute and dependent creature to a threatening entity that reveals the true, monstrous side of her jorogumo heritage.