Tilly is a 15-year-old girl living on the Devil's Backbone in Grand Tower, Illinois. As the most reliable child in her family, she manages the bulk of the household chores and absorbs her mother's anxieties about the impending Civil War. She is cautious and practical, but the sudden arrival of extravagant strangers makes her recognize her own desire for a broader world.

Delphine is a flamboyant, beautiful 15-year-old traveler from New Orleans who seeks refuge in Grand Tower. She dresses in expensive fabrics, speaks with a French accent, and carries herself like a stage actress. Her bold confidence and wealthy habits deeply disrupt the quiet, impoverished routine of the Pruitt household.

Noah is Tilly's 15-year-old twin brother, a quiet and restless young man who works at the local hardware and ship repair shop. He feels the pull of the impending war and actively trains to join the Union Army. The arrival of guests temporarily anchors him to the house, bringing out a surprisingly talkative side to his personality.

Mama is the tough, overworked matriarch of the Pruitt family. Abandoned by her husband years ago, she raises her three children in poverty on the Devil's Backbone while fiercely guarding them against the town's judgment. She is particularly possessive of Noah, living in constant fear that he will leave her for the river or the war.

Cass is the frail 12-year-old sister of Tilly and Noah. She possesses a supernatural ability to see visions of tragedies, both past and future, which leaves her physically exhausted and socially isolated. She spends most of her days wandering the woods or staring at the Mississippi River until a new companion helps draw her out of her shell.

Calinda is a quiet, dark-skinned young woman who arrives in Grand Tower alongside Delphine. Wearing intricately knotted headwraps called tignons, she initially remains silent and industrious. She takes charge of the Pruitts' cooking and introduces them to Creole dishes like jambalaya, displaying a steady presence and an extensive knowledge of natural remedies.

Dr. Hutchings is a polite, 25-year-old physician practicing in Grand Tower. He stands apart from the rougher men of the town through his education and refined manners. He treats the local residents and shows a gentle, courteous interest in Tilly when they cross paths.

Howard is a 15-year-old boy living in 1916 on the brink of American entry into the Great War. He knows almost nothing about his father's side of the family and struggles to imagine the ancient history of the Civil War. During his week-long visit to Grand Tower, he listens to the stories of his elders to understand his heritage.

William is a busy, well-respected doctor practicing in St. Louis in 1916. Anticipating the United States' entry into the Great War, he decides to take his sons on a road trip to visit his childhood home and elderly relatives before he enlists as a military doctor.

Raymond is Howard's five-year-old twin brother. Energetic and rowdy, he travels from St. Louis to Grand Tower, where the sheer weight of the homeplace's history temporarily quiets his usual behavior.

Earl is the five-year-old twin brother of Raymond. He joins his father and older brothers on the journey to the Devil's Backbone, trailing after the older relatives once they arrive at the historic house.

Curry is a young man in Grand Tower who works at Rodgers's store. He supports the Southern cause and spends his free time drilling with other local boys who intend to join the Confederate Army, creating political tension with his Northern-leaning peers.

Paw is the absent patriarch of the Pruitt family. He left Grand Tower years ago to work on the river and log in the north, never returning. His long absence leaves a void in the household and places an enormous burden on his wife and children.