She is William's daughter, and he believes she blames him for her mother's death.
Emily was present at her grandfather's retirement party with her mother and father. She approached Dolores, who had been taken out of the park to play the piano at the event. Emily told her that she was pretty. When Dolores asked her name, Juliet called Emily over to her to stop her interacting with the host.
William, as the Man in Black, is Emily's father; he believes she hates him because she blames him for her mother's death. It's not known what their relationship was like before the death of her mother.
Emily appears to have loved her mother very much, as she resents her father for causing her death. She was very perceptive about her mother's moods and feelings, as she picks up that her mother was afraid of her father.