In Monsters, Inc., the little human girl who enters Monstropolis is known as Boo.
Boo follows Sulley through a door at Monsters, Incorporated and accidentally crosses from the human world into the monsters’ city. Because monsters believe human children are toxic and dangerous, her presence creates an emergency inside the factory and forces Sulley and Mike to hide her.
“Boo” is the nickname Sulley uses for the child. The film does not prominently establish a full formal name for her, so “Boo” is the canonical short answer. Mary Gibbs provides Boo’s voice in the original English version, and much of the character’s dialogue consists of childlike sounds rather than conventional spoken lines.
Boo’s friendship with Sulley becomes the emotional center of the story. Her laughter also helps reveal that human children are not poisonous and that laughter can generate more energy than screams.