In the first film, Boo is a 2-year-old human child who has escaped from her room from which Randall intended to kidnap her. Much of the film's plot follows what happens as Sulley and Mike try to get Boo back to safety. When she first appears, Boo is very attached to Sulley (calling him "Kitty"), but he is not so keen.

At the end of Monsters, Inc., Mike and Sully successfully returned Boo back to her bedroom, but her door was shredded so that there wouldn't be any more escapes. However, thanks to Mike's collecting all the door pieces and putting them back together, Sully was able to revisit his young friend again.

In the credits for the movie as well as the screenplay, his name is spelled ''Sulley'' with the 'E'. But the general spelling is ''Sully'' without the 'E'. PIXAR added the 'E' on purpose to distinguish him and make him unique.

Before it was decided that Boo would be a young girl, numerous ideas were considered for her character. At one point she was going to be African-American, or perhaps even from Ireland. She was also much older, and had a backstory that had her enduring endless pranks played on her by her four older brothers.