Illia, also known as Ilias Fragment, is a supporting character who serves as one of the magical grimoires wielded by the protagonist. Her true form is a legendary magical tome, comparable to the Astil Manuscript, and she is the third grimoire acquired by her master. While her actual age is unknown, her physical manifestation is that of a sixteen-year-old girl.

Before the central Breakdown Phenomenon that sets the story in motion, Illia was in the service of Hijiri Kasuga, the protagonist’s cousin. In that role, she acted as a substitute for Hijiri whenever her magical duties required her to be away from home. During these times, Illia would transform to resemble Hijiri and take on her responsibilities, such as cooking and performing household chores to care for the young Arata Kasuga. Illia has access to the memories and appearance of Hijiri, allowing her to perform this role seamlessly.

Following the Breakdown Phenomenon, Illia received new instructions from Hijiri. Her role shifted from caretaker to guide, albeit with a hidden motive. She led Arata to the interdimensional space housing the Royal Biblia Academy, where he was to become a mage. At Hijiri’s behest, Illia initially sought to confine Arata within the academy, suggesting they remain together there until Hijiri could fulfill her plan. This plan involved trapping Arata and another mage within a world that Illia herself created. However, she ultimately submitted herself to Arata as his grimoire to help him escape that constructed reality.

Illia’s personality is notably protective and possessive, particularly regarding Arata. She demonstrates a strong, special fondness for him, which manifests as intense jealousy. She is intolerant of Arata spending time with other women and has displayed aggressive behavior toward his female companions, once attempting to eliminate a fellow mage, Mira Yamana, out of jealousy. This aggressive pursuit was only halted by the manifestation of a powerful aspect of Arata’s own demon lord factor. Despite these initial hostilities, her core motivation is to support Arata. After aligning with him, she provides him with magical power and assistance, using her abilities to help him in his endeavors, such as his escape from the academy. Her primary motivation throughout is tied to her bond with Arata, evolving from following Hijiri’s orders to acting on her own feelings for him.

Illia’s key relationship is with her master, Arata Kasuga. She holds a deep affection for him that defines her actions. Her relationship with her sisters, the other fragments that make up her grimoire, is also a notable part of her background. She also shares a connection with her former master, Hijiri, whose instructions she followed for a significant period. As Arata’s grimoire, she works alongside his other magical tomes, such as Sora, another grimoire in his possession.

In terms of development, Illia begins as an antagonist following the instructions of a former master before being won over by Arata, after which she becomes a loyal and indispensable ally. Her role in the story is that of a support-type magical artifact. She is highly skilled in analysis and assistance, with a notable ability to help Arata regulate the flow of his magical energy. Her expertise lies in interpretation and complementing her master’s powers, functioning as a key tool for his magical development and success.