Considering that the book is titled Tuck Everlasting it is a bit goofy that the main character is a girl named Winnie Foster.
Winnie is a young girl that is unhappy with her family. They are hovering, helicopter parents, and they are completely stifling Winnie. She wanders out into the woods one day and discovers a mysterious spring with a very attractive boy next to it.
That boy is Jesse Tuck. He is one of the children of the Tucks. He has immortal life because he drank from the spring water. He and Winnie fall in love.
Angus Tuck is the father of the Tuck family.
Mae Tuck is the wife of Angus and mother of Jesse and his siblings.
Miles Tuck is Jesse's brother.
Winnie's parents are probably not considered main characters at all, but they are important because they motivate Winnie to spend time with the Tuck family.
The Man in the Yellow Suit is the final major character. He works throughout the novel to discover the Tuck family and their secret.