Mary Gibbs, also known as Boo, is a curious and adventurous young girl in the animated film Monsters, Inc. She accidentally enters the monster world, which causes chaos in the city of Monstropolis. Her innocent nature and affectionate personality eventually lead to a friendship with the main characters, Sulley and Mike. Boo's presence challenges the monsters' perception of humans, ultimately leading to a significant change in how the monster world operates. Her role is pivotal in transforming the energy industry in Monstropolis from scream-based to laughter-based power.