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, the latter of which resulted in the death of Headmaster Rocastle when he foolishly disregarded warnings not to approach her and was promptly vaporised. (TV: Human Nature/The Family of Blood) Despite this, Lucy Cartwright's parents sensed that something was off about their daughter, but were apparently murdered by her with an energy weapon before they could impede the Family's plans.

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, wondered if one day the Doctor would 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)

Personality
Unlike the rest of her family, who would often scream, shout or utter violent threats, Daughter of Mine rarely spoke and often displayed a cold, detached, pitiless and remorseless personality. She was just as murderous as the rest of her family and did not hesitate to kill Lucy Cartwright’s parents or Rocastle and even coldly stated that she found killing people “funny”.

She was also somewhat cowardly and was not above running away when the odds turned against her. When Tim Latimer fired a small sample of the Tenth Doctor’s essence and memories from his fobwatch directly into her face, she fled in terror upon witnessing some of the Doctor’s darker moments. (TV: The Family of Blood)

Behind the scenes
Daughter of Mine is one of the enemies in Doctor Who Legacy mobile game.