Song Hwe Ri and Byun Baek Hyun were supposed to be best friends. Byun Baek Hyun thinks they are, but Song Hwe Ri thinks otherwise. Best friends understand each other, and Byun Baek Hyun certainly does not meet that criteria. 

"You've always understood me, but I've never undeerstood you. But I'm ready, I'm ready to give my all to understand you, just like how you did a decade ago."