mochiki:
My guess is Seung Yeob (the baseball coach). They showed him in the first few episodes and suddenly he was no longer on scene much. He's at the places, has the access and fits most of the clues.
I think this too. He is the person who has access to DB, even he isn't with her, through her son, Jong Ryeol and Yong Shik. And has the perfect excuse to know things about her without need to ask ou spy her all the time.
He's connected to Jong Ryeol too, ex-boyfriend of BD and Pil Gu father. And somehow I believe that he knows about the relation between Pil Gu and Jong Ryeol.
In the first episodes, we can see that he's close to Yong Shik, like best friends, after all, YS tells him about the girl he know in the bookstore. And through this friendship he have informations about DB too.
And, if Hyang Mi will be killied by the joker, I believe that motive is: HM discover that he's the Joker, she blackmails him and he kills her.
To me make too much sense. And for u, guys?