Megs is a 17-year-old mathematics and physics scholarship student at Somerville College, Oxford. Valuing equations and science over literature, she feels out of place among her peers. Her deep love for her ill younger brother drives her to step out of her rigid academic comfort zone. She tracks down the famous author C. S. Lewis to ask him about the origins of his new children's book.

George is an eight-year-old boy suffering from an incurable heart condition. Confined mostly indoors, he possesses a vibrant imagination and deep intellectual curiosity. He seeks comfort in the world of Narnia, frequently sitting in his own wardrobe while trying to ignore his grim medical prognosis.

C. S. Lewis, known as "Jack" to his friends, is a 52-year-old tutor of English literature at Magdalen College, Oxford. Despite his intimidating academic reputation, he is warm, compassionate, and closely bonded with his brother. When approached by Megs, he opts to answer her brother's question about Narnia by sharing biographical stories from his childhood and early adulthood.

Warnie is a retired British army officer and the older brother of C. S. Lewis. He shares the Kilns with his brother, acting as his secretary and closest companion. Like his brother, he firmly believes in the power of fiction to expand human awareness.

Mr. Devonshire is a hardworking father who manages the local market in Worcestershire. Deeply distressed by his son's terminal illness, he works extended hours in the desperate hope of finding and affording a medical cure. He initially struggles to understand his son's fascination with fairy tales.

Mrs. Devonshire is a practical, rosy-cheeked farmer's daughter who attempts to maintain a cheerful facade for her ailing son. Secretly devastated by his condition, she distracts herself with gardening. She favors Dorothy L. Sayers mystery novels over the darker themes of C. S. Lewis's stories.

Padraig is an energetic literature student at Oxford, originally from Northern Ireland. With vibrant green eyes and an appreciation for myths, he encourages Megs to view literature as equally valuable to science. He becomes deeply invested in helping Megs fulfill George's wishes.

Reverend Capron is the cruel headmaster of the Wynyard School in England. He unjustifiably punishes his pupils and enforces rigid routines, traumatizing a young C. S. Lewis. He represents the first true example of evil the author encounters in his life.

Known informally as "the Knock," Kirkpatrick is an imposing private tutor who prepares the teenage Lewis for Oxford. An atheist with a sharp focus on debate, he nurtures Lewis's rigorous intellect and love of Greek literature.

J. R. R. Tolkien is an Oxford English professor, a devout Catholic, and a fellow fantasy author. Through long discussions about the nature of myth, he challenges Lewis to write stories they would have enjoyed as children.

Flora is C. S. Lewis's mother, a writer who studied mathematics and physics at Queen's College, Belfast. She passes away from cancer when Lewis is only ten years old, a profound loss that permanently impacts his worldview.

Albert is C. S. Lewis and Warnie's father. He lives at Little Lea in Ireland and decides to send his sons away to boarding school after his wife's death.

Arthur is a boy who lives near Little Lea and suffers from a debilitating heart condition. He and the teenage Lewis become close friends over their shared love of Norse mythology and fantasy literature.

Paddy is Lewis's roommate during their military training. The two young men make a solemn pact to care for each other's families if either is killed in combat.

Mrs. Moore is Paddy's mother, affectionately known as Minto. After Paddy's death in combat, Lewis honors his promise to his friend by taking her in and caring for her.