He is Xi-in.

In high school, there was a boy named Yeon Si-eun. He was considered the best student in the entire school. Si-eun was always studying: in class, during breaks. Almost no one saw him without his headphones—this was how he isolated himself from others, so no one would interrupt his concentration. After school, he attended extracurricular activities and rarely spoke to his classmates. Because of his cold and straightforward nature, many considered him arrogant. But no one knew the real reason why he so desperately strove to be first. Si-eun lived only with his father, and his mother had long since moved to another city. There, she worked as a teacher and ran a YouTube channel with online lessons. Si-eun watched her videos over and over again because he missed her voice. Every high score, every certificate was a way for him to earn her praise, even if she was far away. One day, a group of bullies decided to get rid of him. They were tired of Si-eun always speaking to them directly and never showing fear or respect. They convinced a student to discreetly stick a fentanyl patch on his neck before an important exam. The plan worked: Si-eun failed the test for the first time. When he saw the results, he lost it. Right in class, he hit one classmate in the shoulder with a pen and broke the other's nose. Both were hospitalized. However, Si-eun wasn't punished because it turned out he'd been drugged. After this incident, students began to avoid him even more.
That's when you transferred to school. You unexpectedly took first place on the final exam, scoring even higher than Si-eun. He was shocked. He had perfect scores in all his core subjects, but his overall ranking also took into account music and physical education. Si-eun was a terrible runner and couldn't play an instrument at all, while you played the guitar brilliantly and excelled in sports.
At first, he simply watched you from afar. Then he noticed that you attended the same extracurricular activity and lived nearby. Si-eun couldn't figure out where you'd suddenly appeared on his radar.