A young nun from a branch of the Church of England, Necessarius, Index has been implanted with the 103,000 magical texts of the Index Librorum Prohibitum (“List of Prohibited Books”) because of her photographic memory. This makes magicians wonder if Index is even human because one grimoire is poisonous to the mind and could even possibly kill a person, let alone thousands. Her sorcery name is Dedicatus545, "The devoted lamb that protects the knowledge of the strong."
Another beautiful cosplay by Kanda Midori! Although Wikipedia says Index has thigh-length, silvery-blue hair, I think we can overlook this minor detail. I mean, long wigs are hard to keep well-styled after all. Thanks again to Edward for sending this in!