Witch Grendel, also known as Queen Auriana in English adaptations, serves as the primary antagonist. She rules from her castle, commanding a domain where she systematically abducts children. These victims are transformed into harpies using a specialized machine, becoming her enslaved minions. Her objective is to populate her realm entirely with harpies. Those harpies who fail her face condemnation to the Cave of Eternal Ice, frozen indefinitely.

She deploys harpies like Allegra to capture new victims, such as luring Jack to her castle. Though Jack escapes her initial transformation attempt, his companion Squeeker Mouse is converted. Witch Grendel exhibits an authoritarian leadership style, frequently threatening her harpies with severe consequences for failure and directly overseeing their operations.

Her magical powers center on a crystal ball. This artifact enables her to observe distant events, imprison adversaries in magical bottles, and teleport individuals. She wields additional instruments: a golden chain employed as a whip in combat and a windmill-powered machine for forced transformations.

When Jack rescues Allegra from the Cave of Eternal Ice, Witch Grendel transports them back to the transformation pit and attacks physically. The destruction of her crystal ball by Jack's allies neutralizes her powers, causing her to revert to her true form—a large, goblin-like creature. In this diminished state, she attempts escape via an airship rigged with a time bomb intended to destroy the castle. Harvey Harpy attaches the bomb to her airship, causing a mid-air collision with the castle and her death in the explosion.

Her demise breaks all enchantments, reverting every harpy back to their original human or animal forms and liberating the enslaved children. In her powered state, she possesses distinctive white hair, a trait shared by all her harpies symbolizing their corruption. This visual characteristic vanishes once her curse is lifted.