Index Librorum Prohibitorum, known as Index, is a fifteen-year-old English nun serving the Necessarius branch of the Church of England under the magic name Dedicatus545, embodying her duty as a protector of forbidden knowledge. Her photographic memory safeguards 103,000 grimoires from the Index Librorum Prohibitorum, attracting relentless pursuit from factions coveting this arcane archive. A self-defense mechanism, John’s Pen, activates under threat, unleashing spells like St. George’s Sanctuary and Dragon’s Breath. Should manual override fail, the Feather of Light failsafe initiates a memory reset to preserve her secrets. She wears the Walking Church, a white robe with gold embroidery offering divine protection. Destroyed by Kamijou Touma’s Imagine Breaker, she reconstructs it using safety pins. Despite her encyclopedic grasp of magic, she remains unaware of her own latent power, instinctively countering spells without comprehension. Index displays contrasting traits: courteous and intrigued by technology, yet prone to rash decisions and emotional volatility, especially toward Touma. Misinterpretations frequently escalate into physical confrontations, often resolved by biting. Her attachment to Touma evolves into romantic affection after he sacrifices his memories to save her, though their relationship stalls due to his avoidance of her confession. Pivotal in clashes between magical and scientific forces, she is exploited during World War III when Fiamma of the Right remotely triggers her John’s Pen mode, inducing a rampage marked by crimson wings and mythical armaments like the Sword of Freyr. Freed by Touma, she aids missions against crises such as the Los Angeles disappearances. Her role intensifies during London’s invasion, where she gains a 103,001st grimoire. Confronting Touma’s doppelganger at Windsor Castle, she clashes with Mikoto Misaka. Amid these trials, her journey centers on navigating trust and vulnerability, shaped by Touma’s memory loss and the ethical weight of her grimoire-laden mind.