Gargantua is a significant character in the anime adaptation, originating from a medieval European-inspired world. As a child in an orphanage, he lived alongside Ritsuko and their mute caretaker, Jill. Jill was later revealed to be Eve, the previous guardian of the Great Library, residing temporarily in that world. Overcome by intense jealousy towards Ritsuko for Jill's affection, the young Gargantua accidentally stabbed Jill. Her body dissolved into green particles of energy ("sōma"), drenching both Gargantua and Ritsuko and granting them immortality, halting their aging in adulthood.

Consumed by an obsession to find Eve again, the adult Gargantua mastered dark magic and descended into madness. He was often accompanied by three incompetent demonic henchmen and a servant named Seiren, his appearance marked by distinctive dark blue hair and petrol-colored eyes.

Years later, driven by his desire to locate Eve, Gargantua resurfaced. He kidnapped Princess Mariel, intending to sacrifice her to the current library guardian, Lilith, in exchange for information on Eve's whereabouts. Ritsuko intervened to stop him but was framed for the kidnapping and imprisoned. Gargantua's actions stemmed from his desperate longing and warped attachment rooted in his childhood encounter with Eve/Jill.

His storyline concluded when Eve and the fox spirit Tamamonomae intervened. This led Gargantua to experience a change of heart. He relinquished his destructive pursuit, freed Ritsuko from prison, and took her with him back to the realm Lilith had previously granted him—the same realm intended for Princess Mariel's sacrifice, signifying his redemption and return to isolation.