Bella Yaga is a witch and the primary antagonist who adopts the young orphan girl known as Earwig. She is a stout-framed woman with a striking and flamboyant appearance, characterized by her bright blue, curly hair, which she often ties back when working. She wears heavy makeup, including mismatched eyeshadow, along with elaborate jewelry and vibrant, intricately patterned garments, typically covered by an apron when she is preparing spells. Her past reveals that she was once the drummer in a band named Earwig, a group that also included the reclusive sorcerer Mandrake and Earwig's biological mother. The band eventually broke up, and the reasons for its fracture remain tied to unresolved conflicts that still affect her present life.

On the surface, Bella Yaga presents as cold, selfish, domineering, and ruthlessly efficient. She adopts Earwig under the guise of offering an apprenticeship but does so primarily to secure an extra pair of hands for her demanding and unpleasant spellwork. She imposes arduous chores on the girl, such as grinding rat bones and gathering ingredients in her chaotic, foul-smelling workshop, while refusing to teach any actual magic, insisting that the menial tasks are essential components of her craft. Despite her harsh and cruel demeanor, she is not entirely heartless. She provides Earwig with meals, a private room, and access to a bathroom, showing no signs of physical abuse. Furthermore, she demonstrates an unexpected discomfort with physical affection; when Earwig hugs her, she freezes or appears uncomfortable, suggesting a life of isolation and misery rather than pure malice.

In the story, Bella Yaga's role is to serve as the immediate obstacle for the resourceful Earwig. She commands the household with icy authority, enforcing obedience through threats of conjuring swarms of worms or other hexes, and maintains a fraught alliance with Mandrake. Their relationship is tense, often marked by her criticism of him during meals, which sparks his anger. She also forbids music in the house, a rule that directly reflects the unresolved and bitter feelings stemming from the dissolution of their former band. Her familiar is a talking cat named Thomas, who assists her with spellcasting but ultimately grows to help Earwig in her rebellion.

Bella Yaga is a talented witch who has specialized in brewing potions for her clients, a skill that even Mandrake acknowledges. Her workshop is filled with mystical components like bat wings and pickled rat bones, and she crafts utilitarian enchantments for various purposes, such as sabotaging a ballet rival or ensuring a dog wins a competition. Her notable magical abilities include moving objects with a gesture and manipulating the house itself, such as making doors disappear or reappear with a simple hand movement. When Earwig retaliates against her mistreatment by enchanting her with literal extra hands, Bella Yaga responds furiously by summoning magical worms. This spell fails due to Earwig's innate immunity, leading to chaos that shifts the household dynamics. Over the course of the narrative, fissures emerge in her callous facade. After a significant dispute involving Mandrake and Earwig's cunning stratagems, she is forced into a state of reluctant coexistence. While she remains stern, her earlier rigidity softens, and she ceases to enforce Earwig's labor demands, eventually settling into the new family arrangements after a visit from Earwig's biological mother.