My initial thought was Matthew's, but Myeong Ji will probably try to rip it out of her chest. Then Joo Ran popped into my head, but I have no real reason to think that. Maybe because she's so good with the children.
I've finally settled on Chan Seong. In the last few episodes I think they're going to clean house so to speak and get back down the original pahbos and crazed killer. I'm thinking w/n in the next 2-3 episodes Mo Ne will succeed in killing Chan Seong in a way that won't damage his heart, which will coincidentally be a match. Maybe it'll also be our opportunity to find out why he hates Min Do Hyuk so much. I really never bought that it was because of Mo Ne.