"Make sure you look after her while I'm gone."

Sunggyus one wish as he went to the army was for Myungsoo to take care of his younger sister, Sohee. As he knew that Myungsoo had feelings for his younger sister, he thought it best that he gave him the responsibility of watching her while he was gone. When things go wrong, Myungsoo's responsibility towards Sohee becomes stronger and he'll do anything to fulfill Sunggyus wish.