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Index is a main and eponymous heroine of Toaru Majutsu no Index. Her full name is Index Librorum Prohibitorum, she is also considered the main heroine of the Magic Side; as Touma is considered the hero of the Science Side. She is a member of Necessarius and has Photographic Memory. She holds within herself 103, 000 magical texts known as grimoires. These grimoires hold magical spells, abilities and arcane knowledge which is the reason why Index is perused by the Roman Catholic Church and the Eastern Russian Orthodox Church.

She and Touma met one morning when Touma found her hanging over his balcony's fence. It was here her life and Touma's would change and the story began. Index came to Japan a year before the start of the series, although she had already lost her memories prior to arrival. This was done as part of Stiyl and Kaori's plan to keep Index away from magicians that would misuse her knowledge, as Japan was seen to be a place with little history of magic according to them. Both Stiyl and Kaori believed it was for the best if Index spent the year running away from magicians as opposed to living through a year full of happiness, as then it wouldn't be as painful (for both themselves and Index) when her memory had to be erased. Index eventually ended up in Academy City as she was being chased by Stiyl and Kaori, because it was once again time to erase her memories. Moving from rooftop-to-rooftop during Academy City's unforeseen power outage, she slips and falls, landing onto the apartment balcony of the ever-unfortunate Kamijou Touma.

Much of her background is currently unknown because of the periodic memory loss. It is not known if 'Index' is her true name or if it is a name given to her because she holds within her the 103, 000 grimoires. It is confirmed that Index was specifically chosen as the vessel for the 103, 000 grimoires because she has the ability to perfectly memorize things and also because she is unaffected by the books' debilitating effects; since she possesses no mana and therefore cannot use the grimoires. Though this does not explain how Index can still somehow use magic. It is thought some sort of "programming" in addition to John's Pen; has been implanted into Index to allow her to use magic without being affected by the grimoires.

Later, after becoming the vessel of the grimoires, she was given partners as a means of protection. Three years prior to the start of the series, she was the partner of Aureolus Izzard, then a year later, Stiyl Magnus. Index's relationship with her partners is unknown because of her periodic memory loss, but it is implied that both Aureolus and Stiyl care for her deeply. In Aureolus' case, he was Index's teacher during his time with her. It is confirmed that Kanzaki Kaori was best friends with Index before Index's memory was erased.

Index is small in stature, has a petite build, and, much to her chagrin, a flat chest. She is supposedly very beautiful, as in the novel Touma made an observation on how doll-like she appeared upon their first encounter. It can be assumed that she is very young. She has thigh-length, silvery-blue hair, and large green eyes. She is also often seen wearing her Walking Church, a white, modified nun's habit with gold highlights. It is an accurate replica of the Holy Shroud of Turin and offered Index ultimate protection from most attacks, however, it was destroyed by Touma's Imagine Breaker. She still wears it, though safety pins are now needed to hold it together. She is noted by Touma to be a "gaikokujin" (foreigner), and it is likely that her nationality is English.

She is often elated by things that an adult would find mundane and is slightly ignorant and curious of modern technology. She is usually gentle and polite with people and has a kind nature, as shown when she picked up a stray kitten and took it home, although Touma initially refused. She can be easily irritated in a similar manner to how a child reacts to something they don't like - especially when it comes to Touma, who usually ends up angering her in some way as a consequence of his own bad luck. He is often bitten by Index as punishment. She also has an extreme appetite as shown when she met Accelerator and forced him to spend all of his money treating her to food. Index has taken a liking to the anime "Magical Powered Super Android Girl Kanamin" and once cosplayed as her.

Index is shown to care deeply for Touma. She currently lives in his apartment where "complications" arise due to Index's hunger. Despite Touma no longer remembering anything about Index due to his memory loss, he still keeps up a facade and Index is none the wiser. Index is incredibly moody and even moreso when Touma interacts with other women. Her rival in gaining Touma's affection can be considered to be Misaka Mikoto and later Itsuwa, the latter actually joining Touma's household which complicates matters even more. Despite her innocent, child-like behaviour, Touma still cares for her and despite his memory loss and being oblivious to her feelings, Index will forever remain by his side.

Index, having the 103, 000 at her disposal, can cast a variety of high-tier magic. Her magical abilities include: John's Pen, St. George's Sanctuary, Dragon's Breath, Eli Eli Lama Sabachthani, The Deep Red Stone of Pakus Chavura and Rain of sulphur that burns the land. Index is also know to be able to summon mythological weapons such as the Sword of Freyr and The Sword the Harvest God. In addition to this, Index is also capable of casting spells that do not cost her the use of mana in the forms of Spell Intercept and Sheol Fear. Amongst the many grimoires she has in her possession, only the following have been named:
- The Book of Eibon
- The Lesser Key of Solomon (Lemegeton)
- The Book of the Dead
- Necronomicon
- The Cannabalism Ritual Book
- The Nameless Book
- Momotarou
- The Tetrabiblos
- Baopuzi
- Book of the Law
- The Moon and the Rabbit
- Time related to Life and Death
The Book of Law is actually written by Aleister Crowley. According to both Laura and Stiyl she can only memorize the undeciphered code within the book and cannot interpret it.

During the World War (World War III) Index accompanies Touma and is left in the care of St. George Cathedral - the Necessarius headquarters, Index grows "Red Wings" and defeats Stiyl Magnus with ease, who tries to restrain her in vain. She succeeds in destroying the headquarters in the process. Her heart, shattered when she is told by Stiyl that Touma is nowhere to be found after the fall of the 'Star of Bethlehem' into the Arctic Ocean.