Arnie "Yash" Yashenko is an eighth-grade star athlete who routinely practices with the high school sports teams. After discovering he lacks the physical education credits to graduate middle school, he is forced to attend a remedial summer gym class known as "Slugfest." He is highly competitive, protective of his athletic status, and initially views his summer school classmates with disdain because they lack his physical abilities.

Cleo is a former athlete who quit sports entirely after breaking her foot in a skiing accident. She attends summer school to make up all her eighth-grade classes, including physical education, following a three-month absence. She fills her free time by joining the theater club for a production of a science-fiction musical and frequently calls out Yash over his arrogance.

Arabella is a determined student enrolled in both the remedial gym class and an investigative journalism course. Following her parents' divorce, she developed an inflexible dedication to fairness and truth. She immediately questions her gym teacher's qualifications and begins a relentless crusade to expose her, applying the rules of journalism without considering the broader context.

Mrs. Finnerty is the cheerfully unconventional teacher assigned to oversee the remedial summer gym class. A former second-grade teacher, she structures her physical education curriculum around elementary school playground games and baking activities. Her maternal approach and lack of traditional sports drills cause her students to suspect she lacks the proper teaching credentials.

Kaden is recognized as the smartest student in the eighth grade but completely lacks physical coordination. He attends the remedial summer gym class because he skipped physical education during the regular school year to avoid being mocked in the locker room. Despite his lack of athleticism, he bravely volunteers to help Yash run football drills during their breaks.

Jesse is the school prankster, forced to attend summer school as a punishment for a stunt that flooded the school and a nearby store. Enrolled in both the gym class and investigative journalism, he focuses his reporting project on spreading a fake rumor about the town banning toilets to see how quickly the public will believe misinformation.

Nate is a highly skilled new football player who joins the local high school's summer workouts. His athletic abilities rival Yash's, making him an immediate threat to Yash's position as the starting quarterback. He spends time studying football strategy at the local library and faces heavy pressure from his father to perform at an elite level.

Sarah is a student in the remedial gym class who frequently suffers the fallout of her brother's practical jokes. Deeply appreciative of Mrs. Finnerty's understanding and kindness, Sarah joins the investigative journalism class specifically to monitor Arabella's investigation and protect their gym teacher from being fired.

Hammond is an older high school football player and one of Yash's friends. He regularly mocks the uncoordinated students in the summer gym class. He quickly embraces the new quarterback, Nate, leaving Yash feeling abandoned by his former teammates.

Amir is a high school football player who socializes with Hammond and Yash. He participates in the local mall football games and attends the middle school open house, where he jeers at the summer school students as they attempt an obstacle course.