Index Librorum Prohibitorum is a polarizing figure. On the one hand, she is A Certain Magical Index's main heroine, plays a significant role in linking this universe's magical and scientific sides, often seems to be Touma Kamijou's principal reason for existing, and has a cute design. Conversely, Index does not develop much throughout the light novels or anime adaptation and, when it comes to fan support, she is overshadowed by Misaka Mikoto.
With 103,000 magical grimoires stored in her head, Index needs to be kept out of the hands of magicians who would abuse her powers. Although quite a lot about Index's past has been kept a mystery, there are a few interesting tidbits to learn about Kamachi Kazuma's character.
The focus will be on the anime, as the adaptation does not always go into great detail about facts that are covered thoroughly by the light novels.
10 The First Character
In 2007, MediaWorks released Toaru Majutsu no Index no Subete, the official guide book to the light novels. In one of the interviews included in the book, Kamachi Kazuma revealed that Index was the first character envisioned for the light novels and that Touma was written in response to her.
While Touma might be the main character, Index acts as the inciting incident that gets the story moving. Nothing that happens in the main plot would take place without Index, so it makes sense that she was the first character to be imagined.
9 A Name Born Out Of An Encyclopedia
Considering she is essentially a walking library with a healthy set of chompers, Index's name fits the character. That said, it is still quite an unusual title for a heroine, especially since it lacks much of a personality.
In an interview, Kamachi explained that Index's name was literally taken from an encyclopedia, although it was the kanji term that served as the inspiration.
8 A Temporary Main Character
Despite her name remaining on the tin, Index's role decreases substantially following the opening chapters of A Certain Magical Index. Once New Testament: Toaru Majutsu no Index debuted in 2011, Index was firmly relegated to a secondary character, only regaining her status as a main character for a handful of arcs.
While Index is undoubtedly crucial to the story, other female characters can take her place as the main heroine if the arc calls for it. This is also reflected in the anime, as Index has less of a presence in season 2 and 3.
7 Immature & Mature Beyond Her Years
For someone who is meant to be around 15 years old, Index tends to act like a child. Initially, it might seem like the character's immaturity and naivety are possibly due to a sheltered upbringing; however, in reality, Index only remembers the last year of her life.
In some ways, Index acts beyond her age, as she shows a level of empathy that is quite impressive for someone who is effectively experiencing the world for the first time. Considering she is also constantly in danger, some of Index's more infuriating traits can be forgiven.
6 A Love Bite
Speaking of infuriating habits, Index constantly biting Touma is probably the character's most infamous trait. It happens so frequently in the anime that it paints Index as an ungrateful freeloader who is borderline abusive to someone who regularly risks his life to protect her.
It should be noted that Index never bites anyone besides Touma, even people who deserve it. Taking into account her insomnia and stunted development, biting might be Index's way of showing her affection for Touma, as it is undeniably an intimate activity.
5 A Magic God
As a member of Necessarius, aka The Church of Necessary Evil, Index was chosen to memorize the 103,000 grimoires. This was done not only to preserve the magic contained in these books but to also provide a means to cancel spells if a war ever broke out between the different churches.
Index is essentially a neutralizer. She was cast in this role due to her Perfect Memory – allowing her to retain all information – and the fact she could perform this task without suffering any side-effects. With all this knowledge in her head, Index has the potential to become a Magic God, a being who could effectively bend reality to their whim. Naturally, Necessarius would like to stop this from happening.
4 Blocked Mana & Lost Memory
In this universe, magic requires mana, something that few people possess. Despite literally knowing all of the magic, Index does not have any mana at her disposal, so she cannot actively use the spells in her mind. However, Index was not born without mana but had this weakness imposed upon her by Necessarius.
Every year, Necessarius also erase Index's memory to keep her in-check, typically also swapping her partners to ensure she remains relatively isolated and trapped within their cycle of control.
3 A "Safety" Measure
Every year, Necessarius erases Index's memory to keep her in-check, typically also swapping her partners to ensure she remains isolated and trapped within their cycle of control. If for some reason this does not happen, the only logical solution is to obviously kill her.
As a safety measure, Laura Stuart – the Archbishop of the Church of England – cast a spell called Collar on Index that first causes a bad fever and then kills the girl if her memories are not erased. If someone tries to remove Collar, Index loses control of her body as a last-ditch effort to protect the spell. This is called John's Pen and zaps Index of any of her personality.
2 She Is Not Useless
Due to often doing nothing but eating food, demanding food, and biting Touma's head due to a lack of food, Index has gained somewhat of a reputation for being useless, especially in the anime.
Although the lack of mana greatly limits Index's abilities, her understanding of the spells included in the grimoires means she typically knows the best way to target a magician's weaknesses, making her a valuable commodity in any battle. In most cases, Touma's desire to keep Index out of harm's way is the main reason she comes across as defenseless.
1 Divisive
It is no exaggeration to say that Index is a character people tend to love or hate, and an argument can be made that more fall in the latter category. In a 2019 popularity poll ranking the franchise's characters, Index failed to crack the top 10, while she fared even worse in a 2013 promotional event.
For someone originally positioned as A Certain Magical Index's main heroine, Index's reputation is shockingly bad.