What I think is that they somehow had a happy time with each other and had a break-up and they're all good now since NHD's mother said, "I saw him last month". Based on my not-so-logical speculations, maybe Kim Min Chae is adopted or a friend's daughter or something like that which usually happens in K-dramas cause I can't think of anything else at this point, how could it even be possible that the once so lovely and charismatic and talkative Na Hee Do isn't in contact with Baek Yi Jin. What I thought at the start was that maybe they changed their surname and Yi Jin and Hee Do are still married, that's before NHD's mother said "I saw him last month" or was it met? Anyways, it's not possible. Since Min Chae said, "mom's ex-boyfriend", I could think only of this.
About the ending, it's really so unpredictable, the story's good at keeping us at an edge but what I guess it would end like is, Min Chae finding out who her father is (or does she know it already but we don't?) and maybe Yi Jin and Hee Do clearing out what made them break up and get back together or maybe become best friends at least?
What I'm even more curious about now is how Yu Rim and the class seven's pretty boy's relationship is going to end up like and what's going to happen with the school's no. 1 girl aka Seung Wan? Any theories or guesses?