Index was raised within Necessarius, where she was trained to store and safeguard forbidden magical texts. Due to the immense danger posed by the grimoires, the church placed strict limitations on her life, ensuring that she remained under constant surveillance. She was led to believe that her mind needed to be reset every year to prevent the grimoires from corrupting her, though this was later revealed to be a manipulative lie to keep her under control.

Before the events of Toaru Majutsu no Index, Index was on the run from the church after learning the truth about her condition. This led to her fateful encounter with Kamijou Touma, who saved her from magical pursuers and helped her break free from the control of Necessarius. However, despite gaining more freedom, she remains a target for various magical organizations, forcing her to rely on Touma and other allies for protection.

Index has long, silver-white hair and emerald-green eyes, giving her a distinct and ethereal appearance. She is petite and youthful, often mistaken for someone younger than her actual age due to her childlike expressions and small stature.

She wears a white nun’s habit with golden trimmings, which signifies her affiliation with the Church of England. Her attire includes a long white robe with golden accents, a headdress covering her forehead, and a pair of simple shoes, reflecting her status as a religious figure. Despite her cute and innocent appearance, she holds dangerous and forbidden knowledge, making her a highly sought-after figure in the magical world.

Index is a cheerful, curious, and highly expressive individual, often displaying childlike innocence and excitement over everyday things. Due to her sheltered upbringing, she is easily amazed by technology and normal social interactions, making her somewhat naïve compared to other characters. She enjoys eating, sleeping, and exploring, often indulging in Touma’s food and taking over his bed without hesitation.

However, despite her kind and bubbly nature, Index has a fierce temper, especially when Touma upsets or ignores her. She is highly emotional, reacting dramatically to hunger, neglect, or misunderstandings, often resorting to biting Touma as punishment. While her reactions are played for comedic effect, they reflect her strong attachment to those she cares about, particularly Touma, who serves as her protector and closest companion.

Beneath her innocent demeanor, Index is highly intelligent and deeply compassionate, willing to risk herself for the sake of others despite her fragile appearance. Her vast magical knowledge makes her invaluable in battles, and she has moments where she displays profound wisdom, especially when guiding or assisting allies.

• Fateful Encounter with Kamijou Touma
  Index’s story begins when she escapes from Necessarius and is hunted by magicians who seek to capture or kill her due to the grimoires stored in her memory. Injured and desperate, she is found by Kamijou Touma, who saves her from her pursuers despite his initial disbelief in magic. This encounter sets Touma on the path of involvement with the Magic Side, as he learns about Index’s importance and the dangers surrounding her. Through his actions, Index is freed from the church’s restrictions, though she remains a target of various factions seeking her knowledge.

• Ongoing Conflicts and Battles
  Despite her frail appearance, Index is frequently involved in major battles between the Magic and Science factions. While she is often protected by Touma, she is not entirely powerless, as she possesses various defensive spells, magical barriers, and the ability to analyze powerful spells in real time. Her most dangerous ability is John’s Pen Mode, where she temporarily loses consciousness and gains divine-level combat abilities, making her a formidable force when activated. However, she rarely uses this ability intentionally, as it is a failsafe mechanism triggered by external forces.

• Serving as a Magical Encyclopedia
  One of Index’s most important roles is her function as a living grimoire library, allowing her to analyze and counter magical threats by drawing from her 103,000 stored texts. She often provides Touma and his allies with crucial information, helping them understand the abilities and weaknesses of their enemies.