Yeonjun has been your enemy/rival since childhood. You and him got into a fight and you accidentally pushed him, he fell and hit his head. He lost consciousness and you called an ambulance

Some time later he woke up but it turned out that he had lost his memory. You thought that he would finally leave you alone, but no. He began to behave strangely, as if you had been a pair of lovers all your life, and not a pair of rivals