Her name is based on the book with the same name, The List of Forbidden Books or "Index Librorum Prohibitorum" (禁書目録 or Kinsho Mokuroku in Japanese). It is a list of publications prohibited by the Roman Catholic Church.
The avowed aim of the list was to protect the faith and morals of the faithful by preventing the reading of immoral books or works containing theological errors, although it also contained scientific works by leading astronomers, such as Johannes Kepler. The various editions also contained the rules of the Church relating to the reading, selling and censorship of books. It was abolished on 14 June 1966 by Pope Paul VI.
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 right hand. 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.
Much of her background is currently unknown because of the periodic memory loss that was brought upon her. Although, Index states that she was born and raised in St. George's Cathedral, there is not enough evidence to corrobrate this, as Index as well is unsure of it.
- It is a recurring gag for Index to bite Touma whenever he has inadvertently annoyed or irritated her in some way, occuring even in serious situations or in front of important persons.
- Index is still wearing her Walking Church, even though it has lost its powers.
- Index's 'Dragon's Breath' is one of the few magical attacks shown so far that is able to surpass Touma's Imagine Breaker's cancelling speed.
- Index is still the only girl to have confessed to Touma, however, Touma avoided answering by changing the theme of the conversation because since he had lost his memories, he had no idea about what kind of relationship he used to have with Index.
- Index is also the first and only girl known to kiss Touma (by the cheek) when she intended to bite him during the Daihasei Festival Arc.
- Index's bust size has increased a little bit as evidenced in one of the illustrations of Volume 16.
- Touma has pointed out that she is a sleep walker, and that will always tuck into the futons of others if she is just left alone. This is the main reason of why Touma locks himself in the bathroom every night.
- During the Angel Fall incident, Touma and Index found out that Index has her own Academy City ID, though neither remembers having applied for it. In the novel Touma thought it was possible using sophiscated equipment to obtain data like fingerprint, retina scans, voice imprint etc, though he doesn't know why anyone would do this. It is implied Aleister Crowley was the one responsible after making an arrangement with the Anglican Church to keep Index inside Academy City. Also, in episode 15 of the anime, Index's ID picture showed for some unknown reason Index holding a green onion. It is currently unknown what the intentions of J.C. Staff was on this matter. Probably a reference to Vocaloid's Hatsune Miku.
- According to the interview on Kazuma Kamachi featured in "Toaru Majutsu no Index no Subete", the author states that Index is the one whom he want to choose as his lover among all Index characters.