The Master of the Labyrinth (Hangul: 미궁의 주인) is the final boss encountered White Day: A Labyrinth Named School and Remake. In the original game she is a spirit/entity of unknown origin who has the ability to posses body's of other people, while in the remake she is the result of the spirit of Seong-a feeding off the negative energy and emotions of the school.
She also known as the Evil Seong-ah.
In the original game the master was trapped inside the School due to the Amulets power, always seeking a way to escape, eventually growing in power and hatred, feeding off the negative energy of the School. In the remake, the master is the spirit of Seong-a, trapped inside the school due to the amulets power, who uses the school's negative energy to grow stronger.
In 1999, Eun-mi, Seong-ah's mother, used the music teacher in a ritual that was supposed to revive her daughter, with Na-young due to be the sacrifice. But the ritual was a failure (either due to the interference of the master not wanting to lose its host in the original game, or a misunderstanding of the amulets and their power in the remake), with Eun-mi going mad due to the excessive force of the ritual.
In 2000, Seong-ah had found So-young and devised a plan to perform the same ritual that was failed years ago. The music teacher however refused her offer because of the guilt he felt for the sacrifice, committing suicide.
On the night before White Day, Seong-a had planned to use So-young as the sacrifice, with Ji-hyeon being the one who could break the seals, but her plans changed when she realized that Ji-hyeon was unreliable, instead looking towards Hui-min to break the seals, unknowingly doing her bidding.
At the end of the game, once the spirits are set free and the amulets power keeping the ghosts trapped is released, Seong-ah either falls after regaining her senses, or is pulled down with the help of Na-young in the remake in order to save her sister.