Aphasia

Daughter/Sister of Mine was a member of a criminal family known only as the Family of Blood.

Biography
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. (TV: Human Nature) 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 disregarded warnings not to approach her and was promptly vaporised. Joan Redfern believed Lucy's parents realised Daughter of Mine was not their child, and were swiftly killed 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 every year, (TV: The Family of Blood) across several incarnations, including the Seventh Doctor, the Twelfth Doctor, the wheelchair-using Doctor, and the redheaded Doctor. (AUDIO: Shadow of a Doubt)

The Tenth Doctor deduced that the Mortal Mirror wasn't a real mirror because "a girl with a red balloon" wasn't inside it. (PROSE: Martha in the Mirror)

Freedom from the mirrors
According to one account, Daughter of Mine managed to escape her imprisonment by possessing Nancy Latimer, and set her father and brother free as well. To free Mother of Mine, they captured the Doctor and Donna Noble in the TARDIS, but the Doctor managed to escape his restraints and trapped the entire Family of Blood with Mother of Mine in the black hole, after freeing Nancy from possession. (PROSE: Blood Will Out)

According to another account, the Thirteenth Doctor freed Daughter of Mine from the mirror after some reflection, and returned her to her home planet to live out her life to its natural end. (WC: The Shadow 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 Legacy mobile game.