The Shadow Prince

Rina wakes up in a cell, with no idea of where she is or who she is. She is put on trial because she is seen as a threat, and she was found outside the wall with no memory of her past. Captain Levi then decides to train Rina as a member of the Survey Corps, until she fully regains her memory. Who is Rina? And how come she has forgotten everything prior to being locked up?