The Master of the Labyrinth (Hangul: 미궁의 주인) is the final boss encountered White Day: A Labyrinth Named School and Remake. She is a spirit or an entity of unknown origin who has the ability to posses body's of other people, preferably those who are alive or are in the process of resurrection.
She also known as the Evil Seong-ah.
In 1998, Seong-ah died in a School fire accident, and Na-young blamed herself for Seong-ah's death because Na-young didn't go to the meeting place.
After the accident, Seong-ah's soul was trapped inside the School. She became an evil spirit because of the misconceptions about Na-young and the dark energy of the school.
In 1999, She used a music teacher in the ritual, and Na-young was to be the sacrifice for a ritual that would revive her. But the ritual was a failure, and Eun-mi went mad due to the excessive force of the ritual.
In 2000, She found So-young and She devised a plan to perform the same ritual that was failed years ago. Alas, the music teacher refused her offer because of the guilt he felt for the sacrifice, and he committed suicide.
Thereby she had been around her. Then she found Ji-hyun, who can see her. She planned to break the seal using him, So Seong-ah called her to school. But, she changed the target towards the new student, Hui-min.
She / it is ultimately stopped after Hui-min collects all the amulets once again inside the ritual rift (known as The Labyrinth) and releases the power of the spirits onto the afterlife, Seong-ah falling short as the labyrinth collapses underneath her, sending her falling into an abyss.