1. Winnie Foster: Winnie is a curious eleven-year-old girl living in Treegap at the beginning of the twentieth century. Her inquisitive nature leads her to explore the woods by her family's home, where she meets the Tuck family. Over the course of the book, Winnie grapples with questions of mortality, immortality, and what it truly means to live a rich and meaningful life.
2. Jesse Tuck: Jesse is the youngest of the Tuck brothers, who all appear to be much younger than their actual ages because of drinking from a magical spring that halts physical aging. He meets Winnie in the woods and introduces her to the family's secret. Although tempted by possible immortality, Jesse's caring and empathetic nature allows him to ultimately decide that a limited mortal lifespan is more worth living than unending existence without experiencing growth and change.
3. Mae Tuck: Mae is the wise and loving matriarch of the Tuck family. Like her sons, she possesses the gift of eternal youth. While aware of the advantages of immortality, Mae longs for an authentic life with love, family, and the ability to truly grow and experience change over time. Her story adds depth to the theme of immortality.
4. Angus Tuck: Angus, along with his wife Mae, found the magical Tuck spring many years earlier. He often feels regret and bitterness because his encounter with the water meant giving up the experience of living a fully human life, including raising his infant daughters and seeing them mature. Angus represents the challenges one could face with immortality without wisdom and selflessness.
5. The Man in the Yellow Suit: This mysterious but persistent character is set on capturing the Tucks and revealing their secret to the whole world or, alternatively, keeping the secret for use as his "life insurance" policy. He poses a considerable threat to the Tucks' freedom and becomes a catalyst for some of the main conflicts in the story.