William Winterbourne is a made-up name, so it's hard to explain exactly who he is. But let's pretend he's a character in a story to help explain things like you're five years old.
William Winterbourne might be the main character in a book or movie. He could be a boy or a man, but let's say he's a boy for now. William might have a family, friends, and maybe even some enemies in the story.
In the story, William might have some challenges to face. Maybe he has to fight a monster or save the world from aliens. Or maybe he just has to learn how to make friends or deal with a bully at school.
The story might have a happy ending, where everything works out great for William in the end. Or it might have a sad ending, where things don't go so well for William.
The important thing to remember is that William Winterbourne is just a pretend character in a story. He's not a real person, so we can make up whatever we want about him. But by following his adventures in the story, we can learn about bravery, friendship, and other important things in life.