Here's why:
* Her moral compass: Winnie's strong sense of right and wrong is crucial to the story's resolution. She chooses not to drink from the spring, defying the Tucks' desire for her to join their immortal life. This decision forces the Tucks to confront their own morality and the consequences of their secret.
* Her connection to the outside world: Winnie's connection to the outside world, where she lives with her family and friends, acts as a bridge between the Tucks' isolation and the larger society. Her understanding of the world beyond the wood helps the Tucks realize the importance of living, even if it means facing mortality.
* Her courage: Winnie's courage in standing up to the Man in the Yellow Suit, even when facing immense danger, ultimately leads to the Tucks' safety and allows the Man's motivations to be exposed.
While the Tucks' actions and the Man's greed contribute to the plot, Winnie's choices are the driving force behind the story's emotional climax. It's her moral compass, her connection to the outside world, and her courage that ultimately shape the ending and leave a lasting impact on the reader.