Aphasia

Daughter/Sister of Mine was a part of a criminal family known as the Family of Blood. She appeared to be the youngest member, as she possessed the body of Lucy Cartwright, a little girl (and the youngest of the possessed humans). With her harmless appearence she was able to infiltrate places, like the party 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 existance, visiting her once a year, every year. (DW: Human Nature / The Family of Blood)

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