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 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. (TV: Human Nature/The Family of Blood)

When the Tenth Doctor caught her after her family tried to take his life essence, he trapped her inside every mirror in existence. He visited her once a year, every year. Her brother, Son of Mine, hoped that one day the Doctor could forgive her. (TV: The Family of Blood)

The Tenth Doctor later mentioned "a girl with a red balloon", saying that the Mortal Mirror wasn't a real mirror or she would be inside it. (PROSE: Martha in the Mirror)