Kyubin didn't directly threaten to out him, but he told the school nurse he needed counseling for a mental illness, which is enough for Seunghee to believe he WOULD out him. He also publicly posted a photo of him crossdressing and then tried to get him to crossdress for the school competition, which would have effectively outed him, since Kyubin's friends would recognize him from the picture. I mean, that's bullying.
He explains later that he keeps distance from his classmates because he's developed feelings for some of them before. It hurt so much each time when he realized he had no hope that he just stopped getting close to anyone. His rudeness is a barrier to keep people away. All of the characters get some massive growth especially in the last forty or so chapters. Even the biggest jerk in the story gets some redemption.
I came to the 45th chapter and i donno do i want to continue. Seunghee is just annoying, he is so rude to others and acting like he was bullied, but i dont see it like that. No one even told him they would out him, but called him to hang out with them. He was the one who taught of it as a bullying.
Seungtaek is selfish, while acting like he cares.
I donno, its hard to find some appealing characters :/