Mira is a high school senior and artist who allows others to assume she is a college student. Grieving the recent loss of her Aunt Phoebe, she carries deep anxiety regarding her mother's emotional stability. This concern drives her to accept a ride from strangers when a blizzard grounds her connecting flight in Newark. As she travels, an unidentified person secretly documents her movements in a series of anonymous letters.

Harper is an affluent international relations major at Pomona College. Outgoing and resourceful, she takes immediate action when the airline cancels all flights by renting a white SUV to drive home. She generously invites several stranded passengers to join her, inadvertently assembling the car full of strangers.

Brecken is a talkative college student who projects confidence and affluence. He frequently occupies the front passenger seat or assumes driving duties, taking the lead on directions as the weather deteriorates. His impulsive decisions occasionally create tension among the passengers.

Josh is a quiet business major dealing with a knee injury that requires him to wear a brace and use crutches. He sits next to Mira during the drive, often reading a book and remaining generally observant. He occasionally chimes in on decisions about the route or notes the changing dynamics between the other passengers.

Kayla is a reserved young woman traveling in the back seat of the SUV. She spends much of the early journey sleeping and keeps her thoughts entirely to herself. She wears a medical alert bracelet, prompting quiet concern from some of the other passengers about her overall health.

Mira's mother lives in Pennsylvania and is facing the impending anniversary of her twin sister's death. Her recent, unannounced separation from her husband adds to her emotional distress. Her fragile state causes Mira immense anxiety about leaving her alone for the holidays.

Aunt Phoebe is Mira's deceased aunt and her mother's twin sister. She died of cancer a year before the events of the story. She strongly encouraged Mira to attend an art high school, and her loss continues to heavily impact the entire family.

Mira's father lives in California, where Mira currently attends art school. He provides a steady, grounding presence over the phone, advising his daughter to trust her instincts and offering to arrange help if she ever feels unsafe.

Daniel is Mira's stepfather. He abruptly informs Mira that he and her mother have separated, a revelation that severely heightens Mira's urgency to find a ride home.

A stranger wearing a yellow hat, old work boots, and bearing a scar on his hand. He repeatedly crosses paths with the stranded travelers at various rest stops. He smells strongly of disinfectant, which deeply unsettles Mira by triggering memories of the hospital where her aunt died.

Zari is Mira's best friend. Because Zari's mother works directly with Mira's mother, Zari provides Mira with secondary information regarding the ongoing separation between Mira's mother and Daniel.