Yalkae, a Joker in High School

Baek discourages Insook from meeting Du-su because he wants Du-su to focus on his studies and improve his poor academic record. As Du-su's tutor, Baek is responsible for his educational progress and believes the romantic relationship is distracting Du-su from his studies.

Earlier, Du-su joined the biking club specifically to get closer to Insook, showing how invested he had become in the relationship.

Du-su paints Hocheol's glasses red as a prank, which causes the glasses to break. Baek becomes furious and considers expelling Du-su from the school. However, at a teachers' meeting, Baek surprisingly pleads for forgiveness and manages to save Du-su from expulsion.

The prank was a continuation of Du-su's troublemaker behavior established early in the film.

Hocheol is absent from class the next day because he got into a bike accident. Due to his poor eyesight (from not having proper glasses after the prank), he crashed and broke his leg. Du-su feels guilty upon learning this and visits Hocheol's home.

The broken glasses from Du-su's prank directly caused Hocheol's accident.

After visiting Hocheol and seeing his condition, Du-su apologizes sincerely. Hocheol responds with a speech about striving for a brighter future, which deeply inspires Du-su. This motivates him to take responsibility and start delivering milk to earn money for his family and to help Hocheol.

The guilt from seeing Hocheol injured, combined with Hocheol's hopeful message, becomes the turning point for Du-su's character development.

Du-su's grades improve as he takes on new responsibilities. By delivering milk regularly and helping Hocheol, he develops discipline and a stronger work ethic. This new maturity and time management skills translate into better focus on his studies during tutoring sessions with Baek.

His decision to work and help Hocheol creates the foundation for his academic improvement.

Du-su learns from Hocheol's neighbor that his leg condition is worsening. He visits the hospital to comfort Hocheol and brings a pair of new glasses as a gift. The gesture shows Du-su's genuine care and responsibility for his friend's wellbeing.

Earlier, Du-su caused Hocheol's eyesight problems through the prank, so this gift represents his attempt at redemption.

While delivering milk as part of his new job, Du-su is involved in a traffic accident and gets injured. Despite his own injury, his main concern remains Hocheol's wellbeing, showing his continued growth and concern for others.

This accident tests Du-su's resilience but does not deter his commitment to his new responsibilities.

Although Baek is angry about the prank on Hocheol, he chooses to advocate for Du-su because he sees potential in the student and believes in giving him a second chance. As Du-su's tutor, Baek has developed a connection with him and wants to help rather than abandon him.

This decision reflects Baek's role as a mentor, which was established when he arrived as the new teacher who was once connected to Du-su's family.

The Na family initially schemes to arrange a marriage between Baek and Na Du-ju, the older sister. Through Baek's tutoring sessions, Na Du-ju and Baek get to know each other better. By the film's end, their arrangement leads to an actual marriage, and the family celebrates their union.

The family proposed this match early in the story, and the developing conversations during tutoring sessions built toward this resolution.

Du-su joins a biking club to get closer to Insook, a Moran Girls High School student who works at a convenience store. He hopes that by joining the same activity, he can spend more time around her and develop their relationship.

This is Du-su's first major action motivated by genuine romantic interest rather than just mischief.