Astrid is a 12-year-old girl with dark, messy hair who lives in Portland. She is sensitive, emotional, and heavily reliant on her best friend, Nicole. After witnessing a roller derby bout, she develops a fierce passion for the sport and signs up for a summer camp, expecting Nicole to join her. When Nicole chooses dance camp instead, Astrid must face the physical and social challenges of junior roller derby alone.

Zoey is a beginner on the junior roller derby team and one of the few girls Astrid's age at the camp. She is a vibrant, energetic theater enthusiast whose bedroom is covered in movie posters, particularly of Hugh Jackman. Friendly and outgoing, she offers Astrid companionship and support in a challenging, unfamiliar environment.

Nicole is Astrid's best friend since first grade. She is interested in ballet, boys, and fashion, traits that increasingly distance her from Astrid as they prepare for junior high. Though she cares for Astrid and has a history of protecting her, she seeks to expand her social circle by attending dance camp instead of roller derby camp.

Astrid's mom is a kind, affectionate university librarian who works hard to provide for her daughter. Finances are often tight, so she relies on her job benefits and careful spending to give Astrid opportunities. She places a high value on education and culture, regularly organizing outings to expose the girls to new experiences.

Rachel is a classmate of Astrid and Nicole who regularly bullies Astrid. She is bossy, focused on social status, and eager to transition into junior high. She attends ballet camp with Nicole and actively tries to exclude Astrid from their growing friendship, frequently mocking Astrid's appearance and interests.

Rainbow Bite is the star jammer for Portland's home roller derby team, the Rose City Rollers. She wears signature rainbow socks and plays with a fearless, energetic style that completely captivates the audience. She inadvertently becomes a distant mentor figure for the junior campers.

Heidi is the main coach for the junior roller derby camp and the Rosebuds team. She understands the physical and mental demands of the sport and pushes the girls hard to prepare them. Though she runs exhausting drills like the "50-Lap Killer," she is observant and encouraging to skaters who show dedication.

Napoleon Blownapart is one of the coaches at the junior roller derby camp. He assists Heidi Go Seek in running drills and teaching the new recruits the fundamentals of the sport.

Adam is a boy who catches Nicole's attention over the summer. He represents the older, junior-high-oriented social world that Nicole is moving toward and that Astrid feels alienated from.

Braidy Punch is a tough player on the opposing junior derby team, the Black Death. She is a formidable physical presence on the track who aggressively blocks the other team's jammers.