The Twin Sister is abandoned by her parents with her twin brother, the protagonist, in a orphanage. While living there, she is tormented by continuous nightmares that show her death and the failures of her brother in the future. When she is 14 years old, she is possessed by a demon who has named itself "Lucifer" and attacks the protagonist while he sleeps, leaving a mysterious mark on his forehead. After the protagonist awakens, the Sister throws the orphanage into a dark realm that leads to a maze, and sits at a throne in its very center. When the protagonist meets her, "Lucifer" asks to choice between his sister or the orphanage. No matter which the protagonist chooses, he will never see her again.
In the New Game Plus, the Twin Sister can be faced as a boss 3 times, in the Center of the Maze, under the Mahoutsukai clan. She is the only one to have Alice's Die for Me! Skill.