She is the young daughter of Steven Shoemaker.
Background
Lily Shoemaker is a 12 year old girl who lives with her older sister Donna Shoemaker and her father. One night she was hosting a sleepover with her friends, and they were playing the game Truth or Dare. She was dared to go into the bathroom and say Bloody Mary in the mirror three times. She does, but nothing happens to her. However, when her father comes down to calm down the noise, he goes back upstairs and you see a figure in all the mirrors he passes. He goes into the bathroom to take his pills, and he is killed by Bloody Mary.
Donna Shoemaker finds him, but his body isn't shown. All that is shown is a massive pool of blood that seems like to much blood, even for an entire human body.