Scenario: Episode 16, one of the very first scenes: Ki Joong's proposes that they date like how normal couples would. She says no, how could they when they're clearly not normal? Let's just stay as friends.
But Ki Joong, so in tune with his girlfriend of five years, senses something amiss. There's more to it that. Is it because of Jung Hoon?
Dong Bi: Nope.
Ki Joong: There's something [you're not telling].
Dong Bi: There isn't.
Ki Joong: There is. (I like how he just knows. Shows men can be intuitive too.)
Dong Bi finally caves, yes there is, she kissed Jung Hoon that day. And I love his reaction: he takes in a sharp intake of breath, like he forgot to breathe as he waited for her answer. Like he was preparing for the worst (she slept with him), but is relieved to hear a kiss was all there is.