The name given to her by Sullivan - is an adorable, vivacious little girl who takes her journey through her closet door into the monster world in stride. She is undaunted by the odd landscape and takes an immediate liking to her new friends Sullivan and Mike. Throughout her experience, Boo remains blissfully unaware of the potential danger she is in. While Sullivan scrambles to return her home, Boo herself is in no particular hurry. She is having way too much fun.