Nova is a twelve-year-old girl in foster care who has autism and is mostly nonverbal. She loves astronomy and eagerly anticipates the launch of the Challenger space shuttle. Because she communicates differently, many adults incorrectly assume she cannot read or understand them. She relies on her imagination, her deep bond with her older sister, and her favorite toy, NASA Bear, to process the world around her.

Bridget is Nova's seventeen-year-old sister and lifelong protector. After being separated from their mother, Bridget essentially raised Nova, teaching her to read and interpreting her unique ways of communicating. She harbors a strong desire to escape the foster care system and dreams of a better, safer life for herself and her sister.

Francine is Nova's current foster mother and a kindergarten teacher. Unlike previous foster parents, she pays close attention to Nova's nonverbal cues and recognizes her intelligence. She actively seeks ways to make Nova comfortable, from buying soft clothing to making customized flashcards.

Billy is Francine's husband and Nova's new foster father. He is a gentle presence in the house who reads aloud to Nova and teaches her how to crack eggs while baking. Like his wife, he believes Nova is highly capable and pushes back against adults who dismiss her.

Joanie is Billy and Francine's adult daughter who is currently attending college. She is warm and accepting of Nova, passing down her childhood toy box and taking her out for breakfast. She treats Nova like a regular preteen rather than a burden.

Mrs. Steele is the social worker who has handled Nova and Bridget's case for five years. She represents the overtaxed welfare system and often displays ableist attitudes toward Nova. She speaks to the twelve-year-old in loud, slow, infantilizing tones and consistently assumes Nova cannot understand what is happening around her.

Mrs. Pierce is Nova's teacher at her new middle school. She is tasked with administering tests to determine Nova's educational level. While she means well, the testing process frequently causes Nova frustration and stress.

Stephanie is a high school student assigned as Nova's buddy. She teaches Nova about supernovas and astronomy, treating her with kindness and validating her extensive knowledge of space.

Mallory is a student in Nova's class. During a home economics cooking assignment, she stands up for Nova when other girls use cruel language to insult her.

Principal Dowling is the head of Jefferson Middle School. Like many adults in Nova's life, he misinterprets her abilities and speaks to her loudly and slowly, assuming she cannot comprehend normal conversation.

Christa McAuliffe is a real-life social studies teacher chosen by NASA for the Challenger mission. Because she is from New Hampshire, just like Nova and Bridget, the sisters view her as proof that ordinary people can achieve extraordinary dreams.