The human child in Monsters, Inc. is named Boo.
Boo is a three-year-old girl who enters the monster world after Randall leaves a door activated on the Scare Floor. She encounters the scarer James P. Sullivan, known as Sulley, and his one-eyed assistant Mike Wazowski. Because monsters believe human children are dangerous and toxic, Sulley initially tries to return her home quickly.
Boo's presence changes the way Sulley and Mike understand their society. Sulley becomes protective of her, while Randall attempts to capture her for experiments involving the Scream Extractor. The machine is designed to collect children's screams, but Sulley and Mike eventually discover the sinister plan and rescue Boo before destroying it.
The name Boo is a nickname rather than the child's formal name; the film does not prominently establish another full name for her. She is also associated with the voice performance of Mary Gibbs, who was a young child during production. Randall is the villain who threatens Boo, while Sulley and Mike are her monster friends, so none of those names identifies the human child.