Circular Prison, Magical Girls, and Witch Trials

[Follows the original story, original female protagonist] She wakes up to find herself locked in prison and suffering from amnesia. She can't make heads or tails of what a witch or magic is. Her fellow inmates are all beautiful, but they don't seem easy to talk to, and the prison itself is nothing special. The guards, in particular, are hard to get along with. Life looks truly miserable. Besides, Otonase Tsumi isn't someone with a special personality. She's not interesting, beautiful, cute, or strong. Compared with her fellow inmates, she seems old-fashioned and ordinary. But... she wants to help others. That's all. (If anything differs from the established setting, that is my own interpretation. I sincerely apologize if it makes you uncomfortable.)