Ridovia is a Catholic nun who first appears in volumes 9 and 10. She has the 'ability' to convert people to Christianity regardless of who they are, criminals, non-believers, or other magicians that share no connection with the Church. She attempts to convert Academy City to Catholics with the help of an artifact called 'Pietro's Cross'.

Index is still a fugitive, and many powerful magical organizations and individuals want to take her to use the books in her memory, eliminating her in the process. War between those organizations is also starting to emerge. Index can only rely on her friend Touma, who has always protected her, along with Misaka and other friends from the Academy. 
 
(Source: Anime News Network)

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Set in a city of not only scientifically advanced superhuman students, but in a world where magic is also real. Kamijyo Touma's right hand, the Imagine Breaker, will negate all magic, psychic, or divine powers, as well as his own luck. One day he finds a young girl hanging on his balcony railing. She turns out to be a nun from the Church of England, and her mind has been implanted with the Index Librorum Prohibitorum&mdash;all the magical texts the Church has removed from circulation. 
(Source: Wikipedia) 
Note: Includes 16 prologues, 18 epilogues, and 6 extras.

Volume 1 (Set between volumes 13 and 14) 
A series of short stories set before the events of volume fourteen including how Team GROUP was formed. 
Volume 2 (Set between volumes 16 and 17) 
A series of short stories including two separate meetings between Touma's and Mikoto's parents, two tales involving two of the Level 5s of Academy City, and other funny stories involving some of the characters. 
(Source: Wikipedia) 
Note: Volume 1 contains a prologue and epilogue.

Science, magic, and a nun on the run! Kamijo and Index return just in time to celebrate A Certain Magical Index’s 10th anniversary with a brand-new season. After their deadly struggle against the Catholic Church and the pending war between magic and science, Kamijo must work harder to protect Index. But with tensions high all around, what new adventures await them in this next chapter?
 
(Source: Funimation)

The story is set in a world where supernatural powers exist through science, and magic exists through religion. 
In the story, Touma Kamijou is an ordinary high school boy with terrible academic scores and a power called Imagine Breaker. Imagine Breaker can defeat any other power, but it also undermines Kamijou's own good luck. His Imagine Breaker's immense strength is literally immeasurable, so hapless Kamijou is listed at the supernatural level of 0. He happens to meet a girl in white hanging off his window balcony. The girl says she is being chased by sorcerers, and Kamijou learns that the girl is a Church of England nun whose memories have been forcibly replaced by Index-Librorum-Prohibitorum &mdash; the 103,000 forbidden texts of the Church.

Lidvia Lorenzetti