Sulley uses the nickname Boo for the human girl in Monsters, Inc. Her actual name is never spoken in the film, so “Boo” is the canonical name audiences and the franchise use for her.
Boo is a young child who accidentally follows an active door into the monster world. Sulley initially worries that her presence will cause panic because monsters believe human children are dangerously toxic. He soon discovers that she is harmless, becomes attached to her, and gives her the affectionate nickname “Boo.”
Boo’s relationship with Sulley changes him from a career-focused champion scarer into a protective friend. Her laughter also helps reveal that the monster world’s energy system is based on a mistaken assumption: laughter produces far more energy than screams. Mary Gibbs provided Boo’s voice, largely through natural reactions recorded during playful interactions because she was very young during production. Some books and reference materials identify her as Mary, but the film itself calls her Boo.