Boo

Boo is a character in Kingdom Hearts III. She appears in Monstropolis, but is originally from an unknown world. She originated in the Disney/Pixar film 

Design
Boo is a toddler with black hair tied into short pigtails and brown eyes. She wears a pink shirt with purple leggings and white socks.

Origin
Boo first appeared in the 2001 film Monsters, Inc., voiced by Mary Gibbs.

In the film, Boo manages to find her way into the Monsters, Inc. company building from her bedroom. Boo is not her real name, but simply, a nickname given to her by Sulley.