Beware of Your Boyfriend

Da-In and Si-Hyun are in the same class, but they've never spoken to each other. One day, an unfamiliar guy appears and says he is the Lee Si-Hyun from the future, and that they'll go out with each other. Da-In doesn't believe him, but as his predictions come true, she starts to trust him. As Da-In shows interest in Si-Hyun, their classmates say they are flirting with each other. However, Si-Hyun has a twin who looks exactly like him. Just what is the identity of this unfamiliar guy?