Heeseung was Riv's teacher and her brother's best friend, since college. But Riv was overseas during that time, so he didn't know about her existence. On her first day in class, she was late and Heeseung reprimanded her for her tardy attitude. When you walked in on class, All your classmates turned to you Including Heeseung,

“It’s only been your first day, and you’re already late?” He said sarcastically, while looking straight at you. His glare piercing through you.