Dean is a 16-year-old high-school junior and a self-described social outcast. When trapped in the Greenway superstore, he assumes the vital role of cooking for the group, making pancakes and enchiladas while caring for the younger children. He has O-type blood, which makes him susceptible to violent outbursts when exposed to the toxic chemical cloud outside. Though not physically imposing, he documents their experiences in a journal and constantly evaluates his low standing among his peers.

Niko is a high-school junior known as the "Brave Hunter Man" because he lives in the foothills with his grandfather. As a Boy Scout with extensive survival skills, he proves instrumental in the early days of the disaster by physically closing damaged AC ducts on the roof. The group eventually elects him as their leader. He has A-type blood, causing him to develop painful blisters when exposed to the airborne chemicals.

Jake is a high-school senior who sits at the very top of the local social hierarchy. Handsome and athletic, he is initially left in charge by the adult bus driver but loses his leadership position to Niko. He frequently relies on his charm and physical presence to assert dominance over the younger boys. To cope with the disaster, he turns to drinking alcohol and taking pain pills he finds in the pharmacy.

Astrid is a beautiful, fiercely independent high-school senior and a champion diver. Accustomed to being at the top of the social ladder, she initially organizes the younger children and prepares food at the store's Pizza Shack. After a frightening incident involving the contaminated water, she isolates herself from the main group, spending her days hiding in the ceiling structure and only coming down at night.

Brayden is a handsome, popular high-school junior whose father works for the North American Aerospace Defense Command (NORAD). Deeply invested in maintaining the high-school social order, he frequently mocks Dean and Niko, using homophobic slurs and physical intimidation to enforce his dominance. He has B-type blood, which makes him prone to paranoid delusions when exposed to the toxic air.

Alex is Dean's younger brother and a highly analytical middle-school student. Known locally for his talent with computers and machines, he uses his intelligence to tackle practical problems in the store. He successfully rigs up a video walkie-talkie system and develops a plan to conserve the store's electricity.

Josie is a 16-year-old student described as a "hippie activist." After spending the first few days withdrawn and in shock from her head wound, she steps up to become a maternal figure for the younger children. She creates comfortable sleeping quarters out of dressing rooms, organizes a memorial ceremony, and commands respect through calm authority rather than intimidation.

Sahalia is an eighth-grade student who desperately wants to be considered part of the older high-school crowd. She frequently wears provocative clothing, using her physical appearance to capture the attention of the older boys. Her desire for social status often puts her at odds with the more conservative girls in the group.

Chloe is a chubby, energetic third-grader with O-type blood. Despite the terrifying circumstances, she tries to maintain order in her own way, meticulously taking notes on the mechanical repairs needed for the damaged vehicles.

Batiste is a deeply religious eight-year-old who considers himself an authority on what constitutes a sin. He proves to be a capable helper in the makeshift kitchen, frequently assisting Dean in preparing large meals like quiche and hash browns for the group.

Ulysses is a quiet, Spanish-speaking first-grader. Finding himself in a frightening environment where few people speak his language, he struggles with the separation from his family until outside survivors arrive.

Max is a first-grader whose mature stories hint at a difficult home life involving alcoholic parents and domestic abuse. He eagerly helps the older kids when allowed, such as assisting Dean with setting up a breakfast sundae bar.

Henry is one of the youngest students trapped in the superstore. He constantly asks for his mother and seeks out protective figures among the older teenagers to keep him safe during the terrifying ordeal.

Caroline is among the youngest children caught in the disaster. Frightened by the strange new world inside the superstore, she clings to her brother and looks to the older students to act as substitute parents.

Craig Appleton is the district's school chancellor who arrives at the Greenway seeking shelter. Injured and exhausted from the chaotic outside world, he provides the isolated students with their first credible information about the government's evacuation efforts in Denver.

Robbie is a Latino maintenance worker from Lewis Palmer High School who arrives at the superstore seeking refuge. He quickly ingratiates himself with the younger children by offering to repair the damaged school bus and speaking Spanish with Ulysses.

Mrs. Wooly is the heroic elementary-middle school bus driver. After rescuing both her own passengers and the trapped high-schoolers, she establishes the initial leadership structure before bravely venturing out into the apocalyptic storm to find help.