Doon is a 13-year-old boy living with his father in the village of Sparks after leading his people out of the underground city of Ember. He is naturally inquisitive, observant, and fascinated by how things work, especially electricity and machinery. Frustrated by the hardships his community faces during the winter, he initiates a dangerous return journey to Ember upon discovering an incomplete manual he believes holds the key to an ancient, helpful invention.

Lina is an adventurous 13-year-old girl who lives with her toddler sister and guardian in Sparks. While she appreciates the wonders of the surface world, she also fears its severe weather, fierce animals, and lack of electricity, missing her former job as a messenger in Ember. Though hesitant about the dangers involved, she puts aside her fears to accompany Doon on his quest to find the mysterious device left by the Builders.

Washton Trogg is a former roamer who has taken up residence in the abandoned city of Ember, renaming it "Darkhold" and styling himself as its powerful leader. He is a rough, imposing man who pillages the empty homes for supplies and treats his family and captives as subordinates. Though illiterate and dismissive of books, he possesses a brutal ingenuity and resourcefulness that make him a formidable threat.

Maggs is a traveling roamer who wanders between settlements trading scavenged pre-Disaster items for food. She lives a difficult, solitary life out of her tiny wagon and possesses crucial items from the ruins, including the mysterious "Directions for Use" book she uses for kindling. Though focused primarily on her own survival, she inadvertently acts as a vital connecting link between various characters.

Kenny is a perceptive young boy from Sparks who has a deep appreciation for the natural world. He prefers observing animals and plants over classroom learning, finding more sense in the workings of nature than in long-lost machinery. Wise for his age and a good judge of character, he quickly pieces together the truth behind Doon and Lina's disappearance and organizes a search.

Originally named Tim, this young boy was taken captive by the Troggs after bandits attacked his family. Renamed "Scawgo" by Washton Trogg, he is kept as an oppressed worker in the abandoned city of Ember. Having a permanently injured foot and lacking his stolen personal treasures, he feels trapped and entirely dependent on his captors.

Torren is Doctor Hesper's nephew, an irritable and somewhat territorial boy living in Sparks. He struggles to accept the newcomers from Ember and eagerly seeks excitement to break up the monotony of village life. Though he joins the search for Doon and Lina hoping for a thrilling adventure, he soon finds that real tracking involves significant discomfort and hardship.

Lizzie is a young girl originally from Ember who is adjusting to life on the surface while recuperating from a winter illness. She harbors a secret crush on Doon and initially romanticizes his and Lina's disappearance, assuming they eloped to start a life together. Despite her fanciful theories, she proves determined enough to join the rescue effort.

Loris is Doon's father, a former resident of Ember who now lives in the Pioneer Hotel outside Sparks. Like many others, he labors hard to help the community prepare for the winter, enduring physical injury and exhaustion. He cares deeply for his son and provides a steady, grounded presence in Doon's life.

Mrs. Murdo is a caring, responsible woman who acts as the guardian for Lina and Poppy. Having taken them in during their time in Ember, she continues to provide stability and supervision for the girls in their new surface life. She expects order and open communication, making Lina anxious about sneaking away without permission.

Doctor Hesper is the primary medical practitioner in Sparks, working tirelessly to treat the community's injuries and winter fevers with severely limited supplies. She shares her cottage with her nephew Torren and takes in Lina, Poppy, and Mrs. Murdo, demonstrating her willingness to help the refugees despite the strained local resources.

Minny is Washton Trogg's wife, a quiet and highly nervous woman who follows her husband's commands without question. Renamed "Minny-Apple" by Trogg to suit his obsession with powerful ancient cities, she spends much of her time tending to the fire and keeping watch over the family's captives.

Yorick is the son of Washton Trogg, renamed after the ancient city of "New Yorick" (New York). He acts as muscle and an obedient assistant for his father, helping to loot the abandoned apartments in Ember and manage the frightened captives.

Kanza is Trogg's daughter, a girl roughly thirteen years old whom he renamed after "Kanza City" (Kansas City). Like her brother, she assists in her father's scavenging operations and helps maintain their makeshift household in the darkened ruins of Ember.

The Chief Builder is the historical architect responsible for designing Ember in the mid-21st century to protect a small population from global disaster. Anticipating the eventual return to the surface, he leaves behind careful instructions and mysterious technological devices intended to help future generations rebuild civilization.