Any theory that I postulate about the denouement of this drama ends up looking like a lumpen sack of incongruous bits. Even though I may have to bury my head in the sand in the end, here I go.
Park, Oh-Ran is cynical and meddlesome, but she has been--possibly unwittingly--in MY's corner at every juncture. I hope she turns out to be a mother figure to MY (whether she be her stepmother or an aunt). Even in Episode 12, she seems to warn MY--ST--KT that the killer is nearby and they should run away from the killer.
Head Nurse Park may be the woman who framed and vilified Do, Hee-Jae and MY, inciting MY's father to commit violence against them. She may have come to the accursed castle as a nurse or an assistant for Doh, Hee-Jae and later started to covet what she shouldn't--Doh's husband and daughter as well as the castle. Coincidentally, the castle is within a short walking distance from the psychiatric hospital, allowing her to "haunt" the castle as a living specter and hold Ko, Dae-Hwan as a hostage as in the movie Misery. She could easily aggravate Ko, Dae-Hwan's health by tampering with his medication. If so, the head nurse is the Witch of the West and the mastermind behind Park, Oh-Ran's antics. Or she could be the mad woman in the cellar with a secret passage.
There is still another possibility--MY may have been adopted (or stolen) and her birth mother may have joined the hospital to bide her time until both her adoptive parents to die before revealing herself to her long lost child. This scenario calls for a villain character who in her manic obsession with MY has been killing those who interfere in her access to or ownership of MY. This possibility calls for Do, Hee-Jae to be an evil stepmother who had gaslighted the young MY and groomed her as a murderess in the making. The stepmother becomes the mother of a character not of a child, who needs nurture and care. I am dying to know who Do, Hee-Jae is and where she has been.
What is the point of the weekdays? And yet, Episodes 13 and 14 may be too raw and painful, which makes me in a way wish Saturday doesn't come too soon.