Akira is a Japanese Norwegian American teenager living in a rural valley in California. She is highly knowledgeable about wilderness survival and deeply concerned about the changing climate, paying close attention to local fire warnings. Though initially shy and preferring the company of her horse to strangers, she is independent, strong-willed, and remarkably observant of the natural world.

Owen is an outgoing, humorous teenager who works as a tour guide for his family's company, Tundra Treks. He originally views the warmer weather and the influx of polar bears primarily as an economic opportunity for his town. He tends to be slightly impulsive and careless, which sometimes frustrates his more serious best friend.

George is an Indigenous teenager from Churchill, Manitoba, whose family has lived in the area for centuries. He is more serious than his best friend Owen and frequently shares traditional Inuit stories passed down from his grandfather. He loves his home deeply but is grappling with the possibility that his family might have to move due to local economic hardships.

Natalie is a seventh grader living in a small, single-story house in Hialeah, Florida. Fascinated by and fearful of severe weather, she spends her time preparing for the "Big One" and imagining a perfect fantasy world called Mariposa. She is observant and increasingly frustrated by the lack of climate awareness among the adults around her.

Sue is a newcomer to California who recently moved from Churchill, Manitoba. Introduced as a seemingly talkative and unprepared stranger to the wilderness, she possesses hidden inner strength. She is a competitive swimmer who quickly finds herself fighting for survival alongside a local teenager when a wildfire surrounds them.

Lars is Akira's father and an experienced outdoorsman who has taught his daughter essential survival skills. Despite his deep knowledge of the Sierra Nevada ecosystem, he firmly believes that natural forces like fires regulate themselves and denies that climate change worsens them.

Tía Beatriz is an elderly neighbor who moved to Miami from Nicaragua after surviving Hurricane Joan. Despite not being biologically related, she is considered family by Natalie and her mother. She represents the vulnerable populations who lack the resources or physical ability to evacuate during major storms.

Patience is an activist and food bank volunteer living in Liberty City, Miami. She is resourceful and dedicated to helping her community, providing displaced residents with immediate shelter and supplies. She previously worked as a babysitter in California before relocating to Florida.

Mama is Natalie's hardworking mother. Lacking the financial resources to leave Miami, she focuses on securing their small, single-story house as best she can. She is fiercely protective of her daughter and quick to take responsibility for her vulnerable elderly neighbor.

Dodger is Akira's beloved horse and trusted companion. He possesses keen animal instincts that allow him to sense danger—like predators and approaching fire—long before the humans around him do, making him an essential partner in navigating the wilderness.

Nanuq is a massive, heavily scarred adult male polar bear. Driven by desperate hunger due to the delayed sea ice, he relentlessly tracks the boys, displaying a curious intelligence that reminds them of local Inuit legends regarding shape-shifting bears.

Daniel is Sue's father, recently relocated to California from Churchill, Manitoba. Unfamiliar with the local terrain and the speed of dry-season wildfires, he quickly finds himself overwhelmed when the flames trap them on the mountain.

Maria Martinez is a local Miami meteorologist whom Natalie deeply admires. She is also a scientist who travels to Churchill, Manitoba, every fall to report on the polar bears and the warming Arctic landscape.

Churro is Tía Beatriz's small chihuahua. Initially hostile and easily frightened, he becomes wholly dependent on Natalie for survival when the hurricane floods their neighborhood and sweeps them away.

Shannon is Natalie's wealthy best friend. Sheltered in a secure building, she views the hurricane preparation casually, highlighting the economic disparities in how the storm affects different Miami residents.

Derek is a Miami resident whose house begins to shift off its foundation during the hurricane. He acts quickly to get his family and Natalie onto the roof, eventually using a baseball bat to break through a skylight.

Anne is Derek's wife. She provides Natalie with dry clothes and emotional support amid the terrifying hurricane surge, treating her with maternal care despite the danger to her own home.

Marcus is one of Derek and Anne's sons. During the flood, he works with his family to stretch off the roof and grab the railing of a passing sailboat.

Javari is Derek and Anne's son. He struggles to survive the chaotic waters when the sailboat they board collides with an apartment balcony in the dark.

Isabel is a Cuban immigrant and mother navigating the hurricane with her children. Having survived Hurricane Andrew in 1992, she provides a sobering perspective on the escalating severity of modern storms.

Valentina is Isabel's daughter, taking refuge on a small drifting boat after her home is submerged by the hurricane.

Ivan is Isabel's son, trapped in the severe weather alongside his mother and sister as they drift through the flooded streets of Miami.

Vicki is a frantic driver attempting to escape the Morris Fire in her vehicle. She stops to offer the injured girls a ride to safety down the mountain.

Hildi is Akira's younger sister who waits at the family home while the megafire approaches from the mountain valley.

Elwood is Lars's horse. He is sent running into the woods ahead of the fire when Lars abandons him to escape in a vehicle.

Boomer is a large male polar bear who wanders into the area and inadvertently aids the boys' escape by starting a fight with the bear stalking them.