* Winnie Foster: The young, curious, and adventurous protagonist.
* Jesse Tuck: The kind and adventurous son of the Tuck family.
* Mae Tuck: Jesse's mother, a strong and loving woman.
* Angus Tuck: Jesse's father, a wise and philosophical man.
* Miles Tuck: Jesse's younger brother, a mischievous and playful character.
* The Man in the Yellow Suit: The antagonist, a sinister and greedy man who wants the secret of the Tucks' immortality.
* The Tree Man: A mysterious and kind old man who knows the secret of the spring water.
* The Foster Family: Winnie's family, including her father, mother, and brother.
There are also a few smaller characters in the story, such as the townspeople and the family who visits the Tucks in the woods.