Index is a 15-year-old English nun serving the Necessarius branch of the Church of England, her name drawn from the *Index Librorum Prohibitorum*—a historical catalog of banned texts. Designed as a living archive, she safeguards 103,000 forbidden grimoires implanted within her mind, her flawless memory preserving their contents indefinitely while shielding her from their corruptive influence. Prevented from wielding magic herself due to a lack of innate mana and a restrictive collar layered with Church-enforced programs, she remains a vault of perilous knowledge others seek to exploit. Her striking silver hair cascades to her thighs, framing green eyes and a petite stature. She dons the Walking Church, a modified nun’s habit replicating the Holy Shroud of Turin’s protective powers. Damaged irreparably by Kamijou Touma’s Imagine Breaker, the garment clings to her form with safety pins—a visual testament to their fateful encounter. The Church enforces annual memory erasures to curb her potential rebellion or ascension as a Magic God, a practice initiated three years before her pivotal meeting with Touma. Partners like Aureolus Izzard and Stiyl Magnus were deceived into upholding this cycle, believing it necessary to contain her power. Awakening in a Japanese alley a year prior with fragmented recollections of her purpose, she gravitated toward Touma, whose defiance of the Church’s control reshaped her destiny. When threatened, her autonomous defense system, John’s Pen, unleashes advanced spells like St. George’s Sanctuary or Dragon’s Breath. This mechanism also generates the Feather of Light—a failsafe to purge her memories if standard methods falter. Though incapable of conscious spellcasting, she dissects and dismantles enemy magic through techniques like Spell Intercept, turning opponents’ strategies against them. Evolving from a fugitive to a determined ally, Index forges bonds that unravel the Church’s lies. Fragmented memories resurface, exposing deeper ties to Stiyl and Kanzaki Kaori. She confronts adversaries like Fiamma of the Right, who weaponizes her grimoires, and briefly manifests angelic wings during a critical clash. Her battles span London’s invasion and a skirmish with Mikoto Misaka at Windsor Castle, cementing her role in thwarting magical crises. Spin-offs depict her comedic pursuits, such as hunting for her lost cat Sphynx or aiding allies in the *Miracle of Endymion* film. Her personality merges childlike curiosity for technology with playful quarrels over her flat chest or Touma’s antics, tempered by a voracious appetite and fondness for the anime *Magical Powered Kanamin*. Reconciling Touma’s erased memories and the Church’s deceit, she strides into the *Genesis Testament* arc, returning to Academy City for a Christmas interlude before venturing to Los Angeles to probe mysterious disappearances. Her path wavers between guarding arcane secrets and chasing ordinary joy—a balance perpetually tested by those coveting the power locked within her mind.