On the surface, he is a neat, bespectacled model student, but whenever he gets caught up in unwanted fights at Yuseong Technical High School—the worst school for delinquents—he unleashes his hidden, formidable fighting skills. He possesses world-class combat power based on Jeet Kune Do and Taekwondo taught to him by his maternal uncle. Unlike his extraordinary fighting skills, he is a tragic hard worker whose grades never seem to improve when it comes to studying. With the sole goal of getting into a good university, he sits at his desk every day studying relentlessly and leads the 'Study Group' with his friends. He has a quirky and bright, innocent personality, but he also possesses a strong sense of responsibility and a kind heart, never tolerating injustice and putting his body on the line to save friends in danger.

She was Yoon Gamin's tutor during her college years and later reunites with him after being appointed as a temporary mathematics teacher at Yuseong Technical High School, the worst school for delinquents. Although she looks like a fragile 'first love' type of teacher, she possesses incredible mental strength and a sense of responsibility, never backing down and speaking her mind in front of the lawless school system and gangster-like students. She is a true 'real teacher' who is the first to understand and support Gamin's intention to fix his studies and the school, protecting students at all costs whenever they are in danger.

The son of Pi Yeonbaek, the head of the nationwide gang Yeonbaek-pa, he is the pinnacle figure who rules over the students by creating a hierarchy system within the school. He appears to be a neat model student and a smart, handsome boy with excellent grades, but he is a dangerous character who hides a cruel and calculating nature behind that smile. He stands at the core opposition, clashing with Yoon Gamin and the Study Group at every turn as they try to protect the school, maximizing the tension of the story. He is the son and primary heir of Pi Yeonbaek, the chairman of YB—a giant corporation that dominates Incheon and was formerly a gang organization. In the story, he is the practical Rank 1 in the Yuseong Technical High School hierarchy.

A realistic character at the delinquent-filled Yuseong Tech who dreams only of studying and going to college, but faces the urgent need for survival due to family circumstances and realistic barriers. He opens his heart to Yoon Gamin's sincerity and persistent persuasion, joining the Study Group as a core member who helps Gamin lead the team. Although he acts prickly and blunt on the outside, he is a reliable ally who secretly looks out for friends in crisis and shows great chemistry while bickering with Gamin.

Skilled in martial arts like Judo, she is a bold character who shows refreshing action without being intimidated, even in the lawless wasteland of Yuseong Tech. While she looks cool and indifferent on the outside, she possesses a warm loyalty, deeply caring for and looking after her friends and younger brother. Even in a school reality that resembles a battlefield, she is a key figure who adds vitality to the story as she moves toward studying and new goals alongside Yoon Gamin.

Lee Jiwoo's best friend, she is an ordinary and fragile student who, unlike the other members, cannot fight at all. Usually introverted and shy, she adds cheerfulness to the story by performing unexpectedly and showing her quirky side during crises or important moments. She is a lovely character who gradually opens up while hanging out with the Study Group members at the lawless Yuseong Tech, quietly lending her strength when her friends are in trouble.

He wants to be good at fighting in the harsh environment of Yuseong Tech, but he has a quirky and clumsy side where things don't go as planned. Captivated by Yoon Gamin's overwhelming fighting skills, he follows him and stays by his side, eventually joining the Study Group naturally. Contrary to his rough first impression, he melts into the team with his humane and cheerful charm, and in crisis situations, he proves to be a reliable strength by exercising persistent mental fortitude.

Pi Hanul's closest associate who rules Yuseong Tech through violence, he is the figure who leads the misdeeds and the atmosphere of fear within the school. Unlike other bullies who are just rough and ignorantly angry, he always carries a relaxed smile while possessing a cruel nature that ruthlessly exploits and torments others. He is a charismatic villain who torments Teacher Lee Hankyung and the Study Group members at every opportunity, creating sharp conflict with Yoon Gamin's faction to maximize tension.

The biological twin brother of the Study Group's action ace, Lee Jiwoo, he is a character who becomes entangled in the center of various incidents and conflicts occurring at Yuseong Tech. By getting involved with delinquent factions or the dark forces within the school, he causes worry for his sister Jiwoo and those around him, often serving as the catalyst for escalating conflicts. Although he initially shows a crooked side, he is a three-dimensional character who grows through a change of heart after experiencing various events and the intervention of those around him.

A first-year student who appears blunt and prickly, drawing clear lines, but hides a deep backstory and scars within. He leads a painful emotional arc as he struggles alone to uncover the secrets and truth behind his mother's death, who passed away unfairly while working as a teacher at Yuseong Tech. He opposes the dark forces of the school, including Pi Hanul, and while he seems cold, he adds tight tension and immersion to the story as he secretly becomes involved with the Study Group members.