Sejeong has a dark past that led her to the state she is in right now - bullying others to feel powerful. Chaeyeon had just transferred to Produce 101 high school, a school for the most talented people. Sejeong was known throughout the whole school as she and her gang (Ki Huihyeon, Kim Chungha, Kim Doyeon), were always picking on people to bully. It might be surprising but out of her gang, Sejeong is the only one who is not academically talented. She's been failing many of her subjects, thus leading to her teacher, Mr. Jung, to ask the top student to tutor her. The top student being the new transfer student. Jung Chaeyeon. She was practically flawless. Personality, looks, brains. You name it, she has it. The only thing that led to her downfall is that she did not know the infamous Kim Sejeong and her gang, leading her to think Sejeong is just a normal kid, with a normal past. Oh how wrong she was... Kim Sejeong had a past that no one would even want to know about. 

And when she finally got Sejeong to warm up to her, she found herself falling for the latter... but it was too late, she had to give her up.