Who overheard the talk secret in tuck everlasting?

In "Tuck Everlasting" by Natalie Babbitt, the character who overhears the secret conversation is a young girl named Winnie Foster. While exploring the woods near her home, she stumbles upon Jesse Tuck and his family, who are discussing their immortality and the implications of drinking from the magical spring. This pivotal moment sets the stage for the central themes of the story regarding life, death, and the choices we make.