Height: 148 cm
Index (インデックス), also known by her full name Index Librorum Prohibitorum (禁書目録, "List of Prohibited Books"), is one of the main heroines and the titular character of the series. She is a young nun from Necessarius (必要悪の教会, Hitsuyouaku no Kyoukai, "Church of Necessary Evil"), a branch of the Church of England. Index is small in stature and has a petite build. About 14 or 15 years old, she has thigh-length, silvery-blue hair with very white skin and large green eyes. She also wears a white robe with golden embroidery sewed onto every edge of it, the Walking Church, that prevents any injury on her, but it is later destroyed by Touma when he touched it with his right hand, which is why her uniform is stitched with golden pins. Her mind has been implanted with the 103,000 magical texts of the Index-Librorum-Prohibitorum as she possesses Photographic memory, which makes magicians wonder if she really is human because just one grimoire is poisonous to the mind, and one is more than enough to kill a person without the proper preparative.
When she is being invaded by outsiders or injured, her self-protection system personality, Johann's Pen (自動書記〈ヨハネのペン〉Jidou Shoki, "Automatic Writing"), will be activated to allow her to use magic to cast high-class magic attacks such as St. George's Sanctuary (聖ジョージの聖域) and Dragon's Breath (竜王の殺息〈ドラゴンブレス〉, Ryuuou no Satsu Iki, "Killing Breath of the Dragon King"). Her self-protection system will use the Feather of Light (光の羽, Hikari no Hane) to force a reset on Index's own memory should a manual reset fail to be performed. Although she does not know that she has 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. Her vast knowledge of the Index-Librorum-Prohibitorum within her mind makes her to be one of the most powerful characters so far and becomes very helpful to her friends in various situations using skills like Spell Intercept (強制詠唱〈スペルインターセプト〉, Kyousei Eishou, "Force Chant"), and Sheol Fear (魔滅の声〈シェオールフィア〉, Mametsu no Koe, "Voice of Destruction of Evil") and makes her a target of interest for many.
She is often elated by a variety of things and is slightly ignorant and curious of modern technology. She usually is gentle and polite with people and has a kind nature, but she is also easily irritated especially by Tōma who usually ends making her mad in some way as consequence of his bad luck which ends with her biting him. She also has an large appetite and uses any excuse to get food from anyone and likes watching an anime series called Magical Powered Kanamin.