Ugly Duckling S4 E3

After confessing her love in elementary school to her crush, Maewnam is literally crushed when he rejects her in front of all his friends. He calls her ugly and it's at that moment that she decides to wear a box on her head. Maewnam is really damaged by those events and she hides behind the box, truly believing that she's ugly. Forced to return to school, Maewnam meets Minton and Zero. Minton is sweet, friendly, and new to the school while Zero is a notorious troublemaker who uses his fists to solve his problems. The question is: who's the boy that called her ugly all of those years ago? Is it Minton? Is it Zero? Is it another person?