Leo is a high-achieving high school senior who relies on a conservative, straitlaced persona to control an unruly part of his genetics he nicknames "McMurphy." After learning the man named on his birth certificate is a legendary punk rocker, Leo's carefully planned future derails. Facing an unfair cheating accusation that threatens his college funding, he takes a grueling job as a junior roadie on a summer music tour to secure Harvard tuition from his presumed biological father.

Melinda is Leo's oldest friend, connected by a childhood tragedy when her father died suddenly. She is a devoted fan of punk and metal music, expressing her liberal ideology through her blog and her intense dark style. Her passion for the band Purge inadvertently sets Leo on his journey, and she follows the Concussed summer festival across the country as a dedicated fan.

Born Marion X. McMurphy, King Maggot is the infamous, aggressive frontman of the legendary 1980s punk band Purge. Known for his destructive antics and raw rage, he sports a shaved head, an electric chair earring, and a hangman's noose. Offstage, he remains unexpectedly sober and aloof, keeping his distance from the intense partying of the festival tour while agreeing to get to know Leo.

Bernie is King Maggot's cousin and the longtime manager for Purge. He initially appears as a welcoming presence on the chaotic music tour, setting Leo up with a junior roadie job and offering guidance. A veteran of the music industry's wild lifestyle, he frequently entertains young women in his hotel room and expects his crew to overlook the heavy substance use on tour.

Owen is a high school student carrying the immense pressure of early expectations to be a genius. He struggles in his honors classes and faces targeted scrutiny from school administration because he is gay. He escapes the academic pressure by following the Concussed punk music festival across the country with Melinda.

Leo's mother is a woman who deeply regrets her brief encounter with the punk rock world decades ago. She relies on massive, complicated jigsaw puzzles—up to 6,000 pieces—to distract herself from stressful situations. She steadfastly avoids discussing the circumstances surrounding Leo's conception and hates that her past is intruding on her son's life.

Leo's father is a grounded presence who raised Leo as his own. He quit his high-stress Wall Street job after the death of his best friend (Melinda's father), a choice that limits his ability to pay for Harvard. He worries about Leo entering the dangerous world of rock tours but firmly advocates for Leo's right to make his own choices and confronts King Maggot about keeping Leo safe.

Cam is Leo's roadie supervisor on the Concussed tour. He frequently assigns Leo the worst physical jobs, subjects him to mean-spirited pranks like pouring beer on him while he sleeps, and bitterly complains that having a teenager around ruins his chances of meeting girls on tour.

Borman is a stubborn high school administrator with a prejudice against Owen. He aggressively pursues an academic ethics violation against both boys, leading directly to the revocation of Leo's Harvard scholarship.

Max is the drummer for Purge. He spends the tour highly stressed about his impending messy divorce, fiercely guarding his drum kit and reluctantly caring for a dog left behind by his vacationing wife.

Pete is a younger guitarist on the Concussed tour who steps in to replace Purge's injured band member. He fully embraces the reckless, substance-fueled lifestyle of a rock star, creating significant trouble for the road crew tasked with keeping him on schedule.

Neb is the original guitarist for Purge. He suffers a hernia during the band's highly anticipated first show back, requiring surgery that sidelines him from the tour and forces the band to find a replacement.

Nicknamed "Gates" due to his computer prowess, Caleb is a member of the high school's Young Republicans club. He harbors an unlikely crush on the liberal, goth-dressing Melinda, admiring her feisty personality.

Fleming is a rigid member of the high school's Young Republicans club who prioritizes the organization's image above personal loyalty. He threatens Leo's leadership position when the academic cheating scandal breaks.

Llama is a standard poodle belonging to Max's soon-to-be-ex-wife. Left in Max's care during the tour, the dog causes chaos, attracts attention at clubs, and becomes the center of a frantic late-night search operation.