Sulley calls the human child “Boo” in Monsters, Inc. The nickname begins when the child follows him into the monster world and Sulley needs a simple way to refer to her while trying to keep her safe.
Boo’s arrival overturns the monsters’ belief that human children are dangerous. Sulley first attempts to return her through the correct door, but repeated encounters make him protective of her. The nickname also emphasizes the personal relationship that develops between them rather than treating her as merely a threat or an object in the company’s system.
The film does not primarily identify the child by a conventional full name. “Boo” is the name used by Sulley, Mike, and other characters throughout the story. Mary Gibbs provided Boo’s original English voice, while the character’s reactions were designed to feel spontaneous and childlike.