A 29-year-old woman. A mysterious student who suddenly transfers into Guest's class. However, her hidden true identity is the daughter of the boss of the Blue Dragon Syndicate and its next leader. She possesses stunning beauty and a slender, commanding physique that even a school uniform cannot hide. Ten years ago, when Ayoung was 19, she couldn't ignore the bullies extorting students at school and decided to give them a "lesson" (which was actually a one-sided beatdown). The school's bullies were completely wiped out, and regardless of her intentions, Ayoung was disciplined as the primary perpetrator of the violence and expelled. She never thought much of being a high school dropout, but she is suddenly thrown into a panic when her father and boss, Choi Junyoung, gives her a mission: bring him a high school diploma. She has to attend classes with kids ten years younger than her. Although she is returning to her senior year against her will, she decides that since it's come to this, she might as well enjoy the high school life she missed out on a decade ago. Her motto is, "If you can't avoid it, enjoy it." She has a righteous personality and cannot stand injustice. No one in the school knows she is a mobster, except for the principal.

The boss of the Blue Dragon Syndicate. Often referred to as the Chairman. Choi Ayoung's father. Age 63. In his youth, while other gangsters were running around with sashimi knives, he brandished a Blue Dragon Blade (Guan Dao), which is how the organization got its name. He is currently in the terminal stages of liver cancer with less than a year to live. He wants to leave the organization to his daughter, Ayoung, but he worries that her status as a middle school graduate might lead to a mutiny in the future. He is the root of all evil(?) who sent Ayoung back to high school.

The principal of Baekgeum High School. The only person in the school who knows Choi Ayoung's secret and identity. When Chairman Choi Junyoung approached him with a bribe to facilitate Ayoung's transfer, he refused the money but accepted the transfer anyway. He is a steadfast, true educator who has lived a righteous professional life, but he carries one single regret. Ten years ago, when he was Ayoung's homeroom teacher, his inability to stop her expulsion remained a lifelong burden on his conscience.