Winnie is a ten-year-old girl living a sheltered, heavily regulated life in the village of Treegap. Frustrated by her strict upbringing, she longs for freedom and a chance to make a meaningful difference in the world. Her curiosity leads her into the nearby wood, bringing her face-to-face with a secret that changes her perspective on life and mortality.

Angus is the patriarch of the Tuck family. He bears the emotional weight of their unchanging existence, viewing eternal life with deep sorrow. He feels stuck while the rest of the natural world moves forward. He possesses a gentle smile and looks upon normal, aging humans with a mixture of envy and deep appreciation.

Mae is a practical, sensible woman who accepts her family's unusual condition without dwelling on what cannot be changed. She wears her hair the same way every day and deeply loves her husband and sons. When her family faces external threats, she relies on decisive, physical action to protect them.

Jesse is the younger Tuck son. He physically appears to be a handsome seventeen-year-old but is actually 104 years old. Unlike his parents, he enjoys his unchanging youth, viewing his condition as an opportunity for endless thrills. He approaches his eternal future with carefree confidence, eager to share his experiences with someone who matches his spirit.

Miles is the older Tuck son, appearing as a solid, dependable young man. Because he married and had children before realizing he was not aging, he carries the pain of losing his mortal family when they grew old without him. He holds a grounded, serious perspective on his condition and feels a duty to do something useful with his endless time.

This mysterious, polite stranger moves rigidly, resembling a well-handled marionette. Driven by old rumors passed down from his grandmother, he holds a lifelong obsession with finding a family that never ages. He uses his refined manners as a mask, hiding a ruthless, elitist determination to claim the secret of immortality for his own profit.

Winnie's household consists of her mother, father, and grandmother, who enforce strict rules on her daily life. They own the wood outside Treegap and maintain a pristine, orderly home. Their restrictive expectations drive Winnie's initial desire to run away, representing the safe but stifling society she wishes to escape.

The Constable is the local law enforcement officer in Treegap. He is a talkative, straightforward man who takes his job seriously but is easily annoyed by the aloof behavior of the stranger he travels with. He represents the rigid laws of human society that fundamentally threaten the Tucks' existence.

The toad is a small, recurring animal living near the Fosters' home. It serves as Winnie's first sounding board when she contemplates running away from her restrictive family life. The creature later becomes an important recipient of her growing protective instincts.