I think it might be Dong Moon. It's starting to look like the usual setting of she's infatuated with Jin at first but then realizes she liked Dong Moon all along (the constant rejection, him saving her etc look like Jung Hee might have a  "I was so blind" moment in the end). That's where my bet is.

Since this is a youth drama, they might develop the puppy love aspect but also focus more on the friendships and how everyone grows up along the way. It seems pretty balanced between the love plotlines and the coming of age story. 

(I'm also super looking forward to see how Young Choon and Hae Joo develop. I sense a lot of angst coming our way with their gangster x girl next door pairing).