Ellie is a twelve-year-old poet who relies on her family's backyard pool as a sanctuary. After splashing people during a childhood pool jump, she faces relentless bullying regarding her weight from peers and her own mother. To cope with the isolation and criticism, she enforces self-imposed "Fat Girl Rules" to avoid drawing attention and limit her physical presence.

Catalina is a musician who writes lyrics and recently moved next door to Ellie. She belongs to a warm, accepting Mexican family that sharply contrasts with Ellie's tense household. Because she does not attend Ellie's school, Catalina offers a fresh slate for a genuine friendship untainted by Ellie's school reputation.

Philip is a psychiatrist and Ellie's father. He recognizes the importance of emotional well-being and stands as Ellie's only solid support system within the household. He attempts to buffer Ellie from his wife's obsessive focus on weight loss, though his decision to arrange therapy for Ellie without her prior knowledge creates brief tension.

Miriam is a journalist who uses her words to expose flaws, an approach she applies directly to her own daughter. She fixates on Ellie's weight as a problem that requires solving through diets and withheld privileges. She dictates Ellie's food choices and ignores the emotional damage her relentless critiques cause.

Liam is Ellie's older brother. He views his younger sister with deep hostility and regularly hurls insults at her regarding her size. He writes hateful entries about her in his journal and bribes her to avoid being seen with her in public.

Anaïs is Ellie's older sister. Years ago, she invented the nickname "Splash," which catalyzed the bullying Ellie endures. While she does not actively torment Ellie alongside Liam and her mother, she remains a passive bystander who looks at Ellie with pity rather than offering intervention.

Dr. Wood is a therapist hired by Philip to help Ellie address her emotional health. She brings her dog to sessions and maintains a steady, non-judgmental presence. She challenges Ellie to draw the hurtful words others say, showing her how to release the pain they cause rather than internalizing it.

Viv is Ellie's childhood best friend who also lives in a larger body. She provides a rare safe space where Ellie feels comfortable dancing and being herself. Her move to Indiana forces Ellie to face the start of middle school without her primary ally.

Marissa is a malicious classmate who leads the charge in ostracizing Ellie. She consistently compares Ellie to a whale and coordinates cruel pranks to humiliate her in front of their peers.

Kortnee operates as the second half of a malicious school duo alongside Marissa. She actively participates in name-calling and eagerly follows Marissa's lead in making Ellie's school environment hostile.

Enemy Number 3 is an unnamed male classmate who routinely snarls insults at Ellie. He projects his own insecurities outward, relying on cruel jokes to deflect attention from himself and maintain a defensive posture at school.

Diana owns a plus-size boutique for teenagers. Inspired by her mother's dedication to sewing custom clothes for her, she creates a welcoming environment that shows Ellie she does not have to change her body to find things that fit.

Nana is Ellie's deceased grandmother. She serves as a positive memory in Ellie's life, having encouraged her to always be herself while subverting Miriam's strict rules to teach Ellie the piano.

Gigi is Ellie's pet dog. The dog provides Ellie with quiet comfort at home and serves as a loyal companion during her most difficult days.