- Mike: "Boo? What's 'Boo'?"
Sulley: That's... what I decided to call her. Is there a problem?"
Mike: "Sulley, you're not supposed to name it. Once you name it, you start getting attached to it. Now put that thing back where it came from, or so help me!"
- —Mike and Sulley
Boo (real name Mary) is a child who accidentally gets into the monster world in Monsters, Inc.
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. Sulley and Mike try hard to keep the child out of sight from other monsters. In "Boo on the Loose", they craft and dress Boo in a monster costume so as not to start a panic. They then proceed to lose her in the park. However Sully develops a bond with Boo and decides its better to repatriate her to her bedroom.
Returning Boo home
Attempting to return Boo home becomes increasingly difficult for Sully and Mike due to their shift, not to mention that Randall is aware she is with Mike. At one point, Sulley, under Mr. Waternoose's orders, then roars like a lion in front of a child used for simulation, not noticing that he has also scared Boo. Sulley tries to apologize to Boo, but she is too terrified of him and runs to cower underneath Waternoose's legs. Waternoose, who has caught sight of Boo, decides to take care of her before anything worse happens. After he and Randall banish Mike and Sulley to the Himalayas, they proceed with extracting screams from Boo by using the Scream Extractor. However, Sulley arrives at the last moment to disable the device to free Boo.
Mike catches up with them, and they rush back to Scare Floor F, with Randall in hot pursuit. Randall chases them to a large chamber where thousands of doors to human world are kept. Mike and Sulley then make Boo laugh to activate the doors, then they go through them, exiting the monster world and entering back into it, in order to lose Randall and reach Boo's door. Just as it looks like Mike and Sulley have lost Randall, Boo suddenly slips off from Sulley's arms, and Randall reveals that he has caught her.
Sulley manages to catch up with Boo, but then Randall ambushes Sulley. Just as Randall is about to finish Sulley off, Boo suddenly jumps on Randall and beats him with a baseball bat, causing him to rapidly change camouflage patterns. Boo has managed to overcome her fear of monsters, especially Randall. After Mike and Sulley banish him through another door, disable that door, and let it fall to the bottom of the chamber to ensure that he will never return, Boo blows a raspberry directed at the shattered door, on the other end of which Randall ends up in the home of a family of rednecks, who mistake him for a gator and beat him unconscious with a shovel.
Mike and Sulley open Boo's door, only to realize that the power has went out, but then, the door moves back to Scare Floor F, where Waternoose and the CDA agents are waiting. Mike runs off with Boo's monster costume as the agents go after him while Waternoose notices Sulley with Boo and her door, and chases them to the simulation room, where he is lured into a trap set by Mike. Mike exposes Waternoose admitting his willingness to kidnap children (possibly including Boo) to Sulley. Mike then reveals to Waternoose that it is a simulation and he was being watched by a crowd including the CDA, who then turn on Waternoose and arrest him.
Immediately thereafter, Roz, revealed to be Agent 001 of the CDA, enters and decides that Boo's door is to be destroyed after she is safely returned to her room in order to ensure that she will never come back again. She lets Sulley have 5 minutes to say goodbye. 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.
Boo and Sulley take a possibly one last look at each other, then after Sulley closes the door, Boo then opens it to possibly surprise him, only to find her closet instead and Sulley nowhere. After her door is shredded, 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 Boo, who (although off-screen) recognizes him by saying, "Kitty!" This gives Sulley a surprised grin on his face as the film closes.
Boo is a nauseatingly adorable toddler who is curious, and naive. She can speak, but has baby, gibberish vocabulary. The only actual words she says in the film are "Boo!", "Kitty!", "Ew" and "Mike Wazowski!". In other ways, she is advanced for her age, such as already being potty trained. This is seen when she shows to Sulley that she needs to go to the bathroom and doesn't need help.
Boo has brown eyes as well as brown hair in pigtails with pink hair ties. She is shown wearing a pink nightshirt, purple capris leggings, and white socks. She makes friends with Mike Wazowski and Sulley after she sneaks through her door into Monsters, Inc.
- In one of the film's early drafts, she was originally an 8 year-old boy named Raymond, who befriended Sulley (back then named Hob).
- According to the artwork in the DVD, there are a bunch of drawings of Boo being slightly older (possibly around the ages 5-10) in her early works; but the final designs were later confirmed that Boo would be a toddler instead of a child. In the documentary with the characters, Billy Crystal said that Boo was an adorable three-year-old little creation.
- At another point, Boo was going to be an only child and befriended Sulley (then known as Johnson) so that he could protect her.
- In Cars, a car version of her named "Boomobile" is mentioned.
- In her bedroom, Boo has a Pixar ball, a rubber Nemo squeak toy (Finding Nemo had not even been released yet at the time), and a Jessie doll.
- In an early animation test for Boo's T-shirt, it has a picture of Heimlich from A Bug's Life on it.
- During the scene where Sulley and Mike hide Boo in the bathroom, one of the songs Boo sings behind the stalls is "Beauty and the Beast" from the animated Disney film of the same name.
- In a book based on the first film, it is revealed that Boo's real name is Mary Gibbs (the same name as her voice actress). In the film, she signs her drawings with the name "Mary."
- According to some Monsters, Inc. coloring pages, some of the outfits Sulley and Mike made Boo wear before deciding on the monster costume was that of a princess, a witch, a clown, and a doctor.
- When Boo tries to open her closet door after Sulley tells her goodbye and closes it, she is now 7% taller since the programmers noticed that she was too short.
- Mary Gibbs was only two-and-a-half years old when she voiced Boo. Because she was so young, it proved difficult to get her to stand in the recording studio and do her lines. Instead, they simply followed her around with a microphone while she played.
- Originally, Boo was going to return in Circle 7's Monsters, Inc. 2 as a teenager. In this film, Sulley and Mike, while trying to celebrate her birthday, find out that Boo and her family have already moved to a different house when they find an old woman sleeping in what was once her room one day, causing the two monsters to travel into the human world to look for her new house. Unfortunately, the two monsters start to forget how to get back to their world, and to make matters worse, Boo doesn't even remember them anymore since teens and adults do not believe in monsters, nor does she think they're funny, and the only way to get back is to try to make her laugh again. The reason why this sequel was scrapped in the first place was because John Lasseter thought that the final shot of the first was just too heartwarming to be continued.
- Some of Boo's cries and squeals are reused in Inside Out for a younger Riley Andersen. In fact, the credits list Mary Gibbs as an additional voice because of this.
- Boo is said to not make any appearance in the upcoming series Monsters At Work because of it being set before the epilogue of the original movie. However, she's mentioned in the 2 episodes The Damaged Room and It's Laughter They're After. In other words, she could actually appear in Season 2 of the series. 
- —Boo, nicknaming Sulley
- "Mike Wazowski!"
- —Boo, reciting Mike's name
- Sulley: "Boo?"
- —Sulley calls to Boo as he looks into her room via her reassembled door
- —Boo, saying her first line.
- "I'm little baby! Kitty! La la la!"
- —Boo running before Sulley.
- "Wanna ride on it! Da da da, da da da!"
- —Boo, saying that she wants to ride on the DVD case.
- "I like that one!"
- —Boo, giving Nemo to Sulley.
- —Boo, as she roars.
- "So get all right!"
- —Boo, as she says to Sulley to get all right.
- "The guy just went into the ball!"
- —Boo, saying to Sulley if Mike has escaped.