Boo

Mary, officially known as Boo, is one of the main protagonists in Monsters, Inc. and Monsters, Inc. 2.

Monsters, Inc.
In the first film, Boo is a 3-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 Mike and Sulley 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. But as the film progresses, he develops a bond with her. At the climax of the film, Boo manages to overcome her fear of Randall by beating him with a baseball bat and roaring at him before Mike and Sulley banish him and destroy the door to ensure he will never return. When Roz decides that Boo's door is to be destroyed, Sulley takes her in to her room and tucks her in after she shows him her toys, including Jessie the Yodeling Cowgirl from Toy Story 2, a ball from Luxo, Jr., and Nemo from Finding Nemo. After her door is destroyed, Sulley feels sad that he'll never see Boo again and keeps one of the splintered pieces from her door as a memento. At the end of the film, Mike brings Sulley (now the new CEO of Monsters, Inc.) a surprise, which turns out to be Boo's door reassembled. The door becomes functional when Sulley places the piece he has kept, and he opens it to see an Boo, who, although off-screen, recognizes him by saying, "Kitty!", giving Sulley a surprised grin on his face as the film closes.

Monsters, Inc. 2
In the second film, Boo is now all grown up.

Trivia

 * In Toy Story 3, there is a girl in the Butterfly Room at Sunnyside Daycare Center who looks like a slightly older Boo.