Boo is the tritagonist of the Monsters, Inc. franchise. She is an innocent 2-year-old human girl who was Sulley and Mike's friend.Her real name is revealed to be Mary Gibbs in one book (which is also the name of her voice actress).Boo called Sulley Kitty since he looked like a blue cat to her.In Monsters University, Boo doesn't appear since the film takes place before Sulley and Mike met her.She is voiced by Mary Gibbs. It is currently unknown who voiced her in the video games.