Kanzaki, Kaori

An eighteen year old magician from Necessarius. Unlike her partner Stiyl, she uses magic to magically strengthen her body. Her magic name is Salvare000. She uses a 2 meter long katana (which actually is a Nodachi) called Shichiten Shichitou (七天七刀), meaning, "Seven Heavens Seven Swords" as her weapon. She has a special attack called Nanasen (七閃), meaning "Seven Flashes", that uses 7 wires that can split into thousands to surround and cut her enemy. She used to be a kind person who treated Index like a sister, but when Index's memories were deleted by Necessarius, Kanzaki took on a cold and silent personality as Index no longer remembered her and consequentially did not trust her. After Touma saves Index's life and her current memories, she begins to feel a great sense of gratitude towards him.

Later in the series, more of her past is revealed. Not only is Kanzaki a skilled swordswoman, but she is a Saint, possessing the "Stigma", the ability to call upon God's power, as well as the former leader of the Amakusa Church. However, Kanzaki left the group as she felt that her incredible luck was in turn bringing misery to the people she cared about in the Amakusa. This led her to despise the concept of luck and fortune. She joined Necessarius in the hopes of changing her fate and befriended Index, developing a sisterly relationship with her. The repeated failed attempts to make Index remember her after each memory erasure resulted in her cold personality as Kanzaki began to accept that no matter how much a person tries, no one can change their destiny. She begins to show a kinder side to herself after Touma saves Index (thus giving her hope that anyone can defy their destiny) and gradually falls in love with him after he helps her face her past with the Amakusa Church, becoming more prone to showing emotion and sometimes showing a little embarrassment if she is with him. She is the protagonist of her own spin-off mini light novel bundled with the Toaru Kagaku no Railgun DVDs.

(Source: Wikipedia, Toaru Majutsu no Index Wiki)