Index

Index, also known by her full name Index Librorum Prohibitorum, is considered the series' main magic side heroine and is a young nun from Necessarius , a branch of the Church of England. Her appearance is that of a young woman with silvery blue hair. She also wears a white robe that is supposed to prevent injury to her (the "Walking Church"), but it is later destroyed by Toma when he touched it, which is why her uniform is stitched with golden pins. Her mind has been implanted with the 103,000 magical texts of the as she possesses, though at the cost of her own memories (or so the Church says). Her sorcery name is Dedicatus545, "The lamb is devoted to protecting the wisdom of the strongest". The Church says that the grimoires take up 85% of her mind, so she only has 15% of memory space left to use in daily activities, which allows her to live normally for less than a year. However, all of this is revealed to be a lie after Tōma begins to question how the Church even got to numbers like 15% and 85% and finds out from Komoe that the human mind can theoretically store up to 140 years of memories. The Church planned to make the grimoires harder to access by telling its underlings to perform "maintenance" on her mind in exactly one-year intervals once her memory has come to its "limit"; that way, she will not remember her close friends and accidentally and/or purposely reveal the contents of the grimoires.

When she is being invaded by outsiders or injured, her self-protection system personality, Johann's Pen mode, will be activated to allow her to use magic. Her self-protection system will use the Feather of Light to force a reset on Index's own memory should a manual reset fail to be performed. Although she naturally does not have any magical powers she can easily identify the types of magic by seeing it or somebody describing it to her and know how to counter them.

After the events of the first novel until the seventh novel, her role within the story changes into a comedic character as two of the recurring jokes about her having a habit of biting Tōma when he makes her angry and her large appetite, much to the chagrin of those who paid for her food. She still remains an important character as the knowledge of the Index-Librorum-Prohibitorum within her mind becomes helpful to her friends using skills like "Spell Intercept" and "Hell Fear" and makes her a target of interest for many. She has strong feelings towards Toma, but they are one-sided since he only sees her like his best friend.