Orsola Aquinas is a major supporting character in the story of A Certain Magical Index II, first introduced during the arc centered on her own rescue. She is an Italian nun who originally served under the Roman Catholic Church, where she worked as an intelligence analysis specialist skilled in deciphering codes and magical texts. She was also an active missionary who preached in non-Christian countries, and her success in spreading her faith across three such nations earned her considerable recognition within the church, including permission to establish a church bearing her own name near Academy City.

Her life took a dangerous turn when it was discovered that she possessed the ability to decipher the Liber AL vel Legis, a grimoire said to contain extremely dangerous magic. Because of this ability, the Roman Catholic Church marked her for death. What the church did not realize was that Orsola's true intention was not to use the grimoire's knowledge but to decode it so that she could destroy the book itself and prevent its harmful contents from ever being used. Forced to flee for her life, she initially sought protection from the Amakusa Church, a Catholic group. However, she soon fled from them as well, because her experiences had left her unable to trust anyone and she feared they only wanted the grimoire's secrets for themselves. As a result, she is first seen in a state of fear and distress, pursued by multiple factions who each have their own interest in her abilities.

Her encounter with Kamijou Touma becomes a turning point for her. Through his sincere efforts to help her, Orsola comes to understand that genuinely good people exist who want to protect her without hidden motives. This realization allows her to let go of her paranoia and reconcile with the members of the Amakusa Church who had been trying to shield her all along. When it is eventually revealed that she never actually succeeded in deciphering the grimoire, the Roman Catholic Church cancels her death sentence. She then receives sanctuary under the Anglican Church and becomes a member of Necessarius, the Anglican Church's internal magical organization, effectively switching her allegiance from the Roman Catholic Church to the Anglican Church. From that point onward, she appears as a recurring supporting character who remains grateful to the people who saved her.

In terms of personality, Orsola is gentle, devout, and deeply committed to protecting ordinary people from the harmful knowledge contained in grimoires. She is humble and polite, often apologizing for the trouble her situation causes others, and she expresses warm and sincere gratitude toward those who come to her aid. Although her ordeal makes her initially cautious and suspicious, she is fundamentally good-hearted and trusting once she is shown genuine kindness. Her primary motivation throughout her story is not personal power but the safety of others, as she believes that dangerous magical knowledge should be kept out of human hands.

Her notable abilities lie in information analysis and cryptography rather than combat. She is an expert at decoding written codes and interpreting magical writings, which makes her a valuable asset as an analyst. She is also an experienced preacher, able to communicate her faith persuasively. Physically, she is a young woman with blonde hair cut in a rounded short bob, blue eyes, and a medium height. She wears the full habit of a Roman Catholic sister, including gloves and a wimple, an outfit that covers her completely and reflects her disciplined, modest character.