Mary Iris Malone, known as Mim, is a sixteen-year-old girl who recently relocated from Ohio to Jackson, Mississippi, following her parents' divorce. She feels deeply isolated in her new environment and resents her father and stepmother. Believing her mother is physically ill and needs her, Mim steals money and boards a northbound Greyhound bus to Cleveland. She views herself as an anomaly, carrying physical and emotional pain that she guards closely from others.

Beck is a college student taking a break from his studies to pursue his passion for photography. He meets Mim on the bus and eventually becomes her travel companion after acquiring a truck. He treats Mim with protective, brotherly care, offering her a sense of safety and grounding during the chaotic events of their road trip.

Walt is an innocent and highly optimistic teenager with Down syndrome whom Mim meets after abandoning the Greyhound bus. Living alone in the woods, he quickly bonds with Mim and joins her journey. His joyful demeanor and lack of cynicism provide a stark contrast to Mim's internal struggles.

Mim's father is a pragmatic man who moves his newly configured family to Mississippi. Haunted by his sister's severe mental health struggles, he is hyper-vigilant about Mim's behavior and insists she see psychiatrists. His attempts to protect his daughter often manifest as controlling behavior, creating a massive rift between them.

Eve is Mim's British mother who remains in Ashland, Ohio, after the divorce. Mim remembers her as a vibrant, spontaneous woman who loved vintage music and hitchhiked across Europe in her youth. Eve's emotional distance and struggles with depression shape much of Mim's childhood, prompting Mim's desperate cross-country journey to reunite with her.

Kathy is Dad's new wife, whom he marries shortly after divorcing Eve. Pregnant and living in the new Mississippi home, she becomes the primary target of Mim's anger over the broken family. Mim views her as an intrusive presence who ruined her parents' marriage.

A middle-aged man wearing a poncho who travels on the same bus as Mim. He presents a respectable outward appearance as a married lawyer, but he exhibits highly predatory behavior toward young girls. He relentlessly stalks and corners Mim during rest stops.

Arlene is an elderly woman who quickly befriends Mim on the bus. She is eccentric, gentle, and entirely accepting of Mim's quirks. She is traveling to Kentucky to visit her estranged nephew, carrying a mysterious wooden box meant specifically for him.

Caleb is a tall, intimidating teenager who camps in the woods near Walt. Having suffered severe abuse from his father, Caleb exhibits highly erratic and violent behavior. He becomes a direct physical threat to Mim and Walt when he decides to steal Walt's money.

Aunt Isabel is Dad's deceased sister. She encouraged a young Mim to journal to manage her emotions and not succumb to the madness of the world. Her struggles with mental illness and subsequent suicide cast a long shadow over the family, particularly driving Dad's intense anxiety about Mim.

Ahab is Arlene's nephew who lives in Independence, Kentucky, where he owns and operates a gas station with his partner. Estranged from his mother due to his sexual orientation, he maintains a connection only with his aunt Arlene.

Albert, whom Mim internally refers to as the "Pale Whale," is Ahab's boyfriend. He helps run the gas station in Kentucky and provides Walt with a hidden key for safe shelter.

Dr. Makundi was Mim's first psychiatrist. Mim adored him because he focused on playing games and telling stories rather than immediately resorting to prescribing medication.

Dr. Wilson is the second psychiatrist Mim's father hires. Unlike his predecessor, Dr. Wilson takes a clinical approach and prescribes Mim the anti-psychotic medication Abilitol, which Mim intensely dislikes.

Claire is Beck's former foster sister. Beck feels a profound sense of failure regarding his promise to always be there for her, a guilt that drives him to seek her out during his road trip.

Michelle is a local veterinarian who agrees to examine Walt when he falls ill and no human hospitals are nearby. She diagnoses his sickness as a severe reaction to MSG.