His ex-girlfriend's sister , Yoon Se - na ( Krystal ) , moves to Seoul in order to pursue her music - composing career .
His ex-girlfriend's sister , Yoon Se - na ( Krystal ) , meets Hyun - wook who helps her achieve it .
His ex-girlfriend's sister , Yoon Se - na ( Krystal ) , moves to Seoul in order to pursue her music - composing career and meets Hyun - wook who helps her achieve it .