Mia is a 13-year-old eighth-grader living in Illinois who keeps a major secret. She has synesthesia. She sees specific colors associated with letters and numbers, as well as spoken sounds. This neurological condition affects her ability to learn math and Spanish, making her anxious about the start of the school year. She finds comfort in painting abstract shapes inspired by Kandinsky. She shares a deep bond with her cat, Mango. She occasionally uses her color associations to cheat on a math test by matching colors to equations.

Jenna is Mia's 13-year-old best friend. She has red hair and freckles. Her mother died of cancer three years ago. Jenna processes the loss by holding onto posthumous birthday gifts and searching for secret doorways in the woods. She wears a fraying friendship bracelet to symbolize her long-standing bond with Mia. She grows highly distressed when people hide medical information from her.

Roger is an eighth-grade student in Mia's history class. He is diligent about schoolwork and takes the lead on group assignments. He frequently attends acupuncture appointments for an injured ankle. He processes his own grief after his family euthanizes their elderly golden retriever at the local vet clinic.

Beth is Mia's 16-year-old sister. She recently returned from her summer trip with an altered personality. She adjusts her daily habits by collecting herbs and adopting a vegetarian diet. She refuses to eat turkey at Thanksgiving. She also stops wearing makeup and ties her hair back in a simple ponytail. She often argues with her siblings but provides support when it matters.

Zack is Mia's 11-year-old brother. He is the only blonde member of the family. He possesses eccentric habits. He maintains a detailed chart of every McDonald's hamburger he eats, which once landed him in a local newspaper. He strictly follows bizarre superstitions like trying to synchronize all the clocks in the house to ring at exactly five o'clock.

Mr. and Mrs. Winchell are supportive figures in their children's lives. The father sells and repairs farm equipment using a personal helicopter. He frequently remodels their sprawling family home. The mother is a former high school science teacher. They initially struggle to understand Mia's color associations but actively seek multiple medical opinions to help her secure a proper diagnosis.

Adam is a 14-year-old boy from Boston. He participates in the synesthesia online discussion group and shares Mia's experience of colored letters and numbers. He encourages her to explore her condition through alternative methods. He suggests she forge a doctor's note to get acupuncture treatments without telling her parents.

Billy is a five-year-old boy living in Mia's town. He possesses synesthesia and freely vocalizes the colors he associates with words. He approaches Mia in a grocery store checkout line to tell her about her name's colors. His mother views his condition as a wild imagination.

Jerry is a neurologist who researches sensory perception at the University of Chicago. He officially diagnoses Mia with synesthesia. He assures her the condition is harmless and lifelong. He connects her to a broader community of synesthetes by giving her access to an online forum and inviting her to an in-person meeting.

Mango is Mia's pet cat. She found him at her grandfather's funeral and named him because his meows look orange to her. He has a chronic wheeze but provides Mia immense emotional comfort. She privately believes a piece of her late grandfather's soul resides inside the cat.

Mrs. Henkle is Billy's mother. She is protective of her son and highly skeptical of anyone validating his colorful vocabulary. She prefers to treat his statements as mere childhood imagination. She acts brusque and dismissive when Mia attempts to explain synesthesia to her at a community food drive.

Mr. Davis is Jenna's widowed father. He attempts to move forward with his life by introducing a new girlfriend. This decision causes friction in his household. He visits the cemetery every Wednesday morning with Jenna to honor his late wife.

Rebecca is the new girlfriend of Jenna's father. She tries to build a connection with Jenna by offering to French braid her hair for an upcoming birthday party. She faces heavy emotional resistance from Jenna, who resents her presence during Sunday family time.

Dr. Randolph is the Winchell family's pediatrician. He admits he is completely stumped by Mia's visual symptoms during a routine check-up. He refers her to a psychotherapist for further evaluation.

Ms. Finn is a psychotherapist who evaluates Mia. She suspects Mia invents the color visions to gain attention as a middle child. She asks probing questions about Mia's family dynamics and potential drug use before referring her to a neurologist.

Samantha is a highly enthusiastic math tutor. She is hired to help Mia improve her failing grades in pre-algebra. Her presence in the Winchell house attracts the attention of Zack, who develops a crush on her.

Faith is an acupuncturist. She treats Roger's ankle injury. Her treatments inadvertently amplify Mia's sensory perceptions when Mia becomes a patient. She uses an electric current during the sessions, which causes Mia to see vibrant colored clouds around people.

Kimberly is a classmate who participates in the girls' social circle. She harbors a crush on a boy at school and expects Jenna and Mia to help her create a love potion.