@Amethyst yeah. She was supposed to be erased. Mok gan's plan is to forced Wi Soyeon body and soul of Cheong Ryeong (blue spirit in her past life) and blue spirit herself (vengeful spirit) to merge to be able to enter reincarnation. If than happens blue spirit's existence will be no more and will be replaced by a new existence and that's Wi So-Yeon. The reason behind this was to easily manipulate her because of mok gan's obsession for Cheong Ryeong.
I don't care about your taste!