Calli is a seventh-grade girl living with Tourette syndrome, obsessive-compulsive disorder, and anxiety. Because her mother repeatedly uproots their family after romantic breakups, Calli longs to stay in one town permanently to make a best friend. Following her doctor's misguided advice, she attempts to hide her tics from peers, which causes her severe emotional and physical distress. She copes by studying astronomy and collecting local rocks from every town she inhabits.

Jinsong is a Chinese American seventh-grader who serves as the student council president at Black Ridge Intermediate School. He is an avid baseball player focused on securing a spot on the local Little League team. Though he immediately likes Calli and builds a friendship with her outside of school, he caves to peer pressure and pretends not to know her in the classroom to avoid being bullied by his friends.

Beatriz is a student at Black Ridge Intermediate School and Jinsong's former best friend. After her mother abandoned her family to move away with her siblings, Beatriz grew bitter and hostile toward her peers. She channels her hurt into bullying behavior, actively leading the classroom in mimicking and harassing Calli for her vocal and motor tics.

Calli's mother is a florist who repeatedly uproots her daughter's life to escape the emotional fallout of her failed romantic relationships. She prioritizes her own emotional recovery and dating life over Calli's need for stability. Misunderstanding Tourette syndrome as a series of bad habits, she pressures Calli to suppress her tics and frequently curtails Calli's coping mechanisms, such as cutting her hair short to prevent pulling.

Duncan is a middle school student who dreams of making the Royals Little League team alongside Jinsong and Nyle. He frequently initiates the verbal harassment against Calli, targeting her unconventional clothes and involuntary noises. He utilizes peer pressure to maintain a cruel status quo within his friend group.

Nyle is part of Jinsong's core social circle at Black Ridge Intermediate School. Together with Duncan, he practices baseball and hopes to secure one of the limited spots on the Little League roster. He actively participates in making Calli's school experience miserable and creates an environment where Jinsong feels too intimidated to speak up.

Tom is a tall, polite man who begins dating Calli's mother shortly after their arrival in St. George. For Calli, Tom represents an impending threat, as she assumes his inevitable breakup with her mother will trigger yet another sudden move across state lines.

Mrs. Baumgartner is the school counselor and psychologist at Black Ridge Intermediate School. She steps in to assist Calli with the social and physical realities of Tourette syndrome. Unlike Calli's mother or previous doctors, she provides a safe environment and encourages Calli to stop suppressing her natural movements.

Kenny is a blue-eyed student in Calli and Jinsong's class. When Calli suffers a painful tic and falls in science class, Kenny is the first student to step forward and offer her help. His willingness to associate with Calli contrasts sharply with Jinsong's early cowardice, setting a standard for actual friendship.

Jinsong's mother is a warm, welcoming presence in their apartment complex. Experiencing morning sickness due to her pregnancy, she relies on Jinsong to help around the house and run errands. She extends hospitality to Calli, stepping in to ensure the young girl is not left alone when Calli's mother goes out.

Chonglin is Jinsong's younger brother. He looks up to Jinsong and exhibits a natural, childlike curiosity about the people around him. During a family picnic, he bluntly asks Calli about her tics, prompting her first open conversation about Tourette syndrome outside of a therapeutic setting.

Dr. Khan is the homeroom teacher at Black Ridge Intermediate School. He accidentally introduces Calli by her legal last name, Snow, and oversees the classroom where much of the initial mocking takes place. He relies on Jinsong to set a strong example as student council president.

Ivy is one of the popular girls at Black Ridge Intermediate School. She dresses and speaks exactly like her friends, forming a tight clique that interrogates Calli on her first day. Her social favor is fickle, and she uses items like friendship lockets to control her peers' standing within the school hierarchy.

Mrs. Locke is the science teacher who inadvertently leaves the room during a critical moment of bullying. Upon returning to find Calli in distress, she takes it upon herself to inform the entire class about Calli's Tourette syndrome, shifting the classroom dynamic and forcing Jinsong to confront his guilt.