Aphasia

Daughter/Sister of Mine was a member of a criminal family known only as the Family of Blood. She appeared to be the youngest family member, as she possessed the body of Lucy Cartwright, a little girl and the youngest of the possessed humans. With her harmless appearance, she was able to infiltrate places, such as the dance hall and the Farringham School for Boys. When she was caught by the Doctor after her family tried to take his life essence, he trapped her inside every mirror in existence, visiting her once a year, every year. (DW: Human Nature / The Family of Blood) Her brother, Son of Mine, hoped that one day the Doctor could forgive her.

Behind the scenes

 * In the book Martha in the Mirror the Tenth Doctor mentions "A Girl With a Red Balloon", saying that the Mortal Mirror isn't a real mirror or she would be inside it.