They were brought together by his best friend and shortly after he found himself falling in love with her, that's the problem though she's his best friends girlfriend. When they break up he doesn't even get the chance to tell her that he loves her because she leaves to study abroad in the states, taking his heart with her and she didn't even know it. They lost contact sometime while she was away and he was focusing on his music, they haven't talked in over a year. 

She's back in Seoul and she's looking for him. 

He's trying to put her in the past and move on.