Boo is a character from the Pixar movie "Monsters, Inc.". She is a young girl who walks through her closet door at night into the world of Monstropolis. Sulley and Mike, the main characters, refer to her as "Boo" because they do not know her real name. However, there is a small detail in the movie that might reveal her real name. One of the many drawings Boo does in the movie has "Mary" written in crayon at the top left corner. It is believed that Boos real name is Mary Gibbs, which is the same name as the young girl who provided the voice for Boo in the movie. Mary Gibbs was not an actor by trade but was connected to Pixar in another way. Her father, Rob Gibbs, worked on Monsters, Inc. as a story artist, and Pixar used Mary as the voice of Boo thanks in part to this connection and since she was relatively the right age to play the character.