There is so little to the plot that each episode chould be summarised using only one or two sentence and you still…
I think it’s because she is the only person who looks directly into his eyes and not afraid of him. She also said she would love him, something that Myeong Mal yearned for his whole life but couldn’t get. Not to mention, TDK is the only person who can see through him, made him realize that he is not a bad person. Maybe these are the reasons for him to fall for her.