Boo is a toddler who is curious and naive. She can talk but has baby gibberish vocabulary, and little dialogue because she's two years old. The only actual words she says in the film are "Boo!", "Kitty!", 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 does not need any assistance.
In the first film, Boo is a two-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 her room safely.
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 one point, Sulley, under Mr. Waternoose's orders, roars in front of a robot 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 the 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. Boo has managed to overcome her fear of 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. They open Boo's door, only to realize that the power has gone 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's plot to kidnap a thousand children (possibly including Boo) to satisfy his own agenda to the agents, who then turn on Waternoose and arrest him.
Immediately thereafter, Roz revealed to be Agent 00001 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 into 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 will probably 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 appears throughout the miniseries, though her largest part is in issues three and four.
In Issue 3, her Jessie doll is stolen by Sid Phillips (though he goes unnamed in the comic), who wants to steal toys from kids to try and spare them from what happened to him. He hides in Boo's room from Mike and Sulley, who have been pursuing him through the door system. After finding her toy has been stolen when he escapes, Boo goes angrily in pursuit, putting on her monster costume and helps Mike and Sulley find Sid in the Monsters, Inc. facility. Getting Jessie back, the monsters don't believe Sid's stories about living toys and send him back to the human world through his sister's door.
In issue 4, Sid, Waternoose, and Randall team up to get their revenge. A visit to Boo's leaves Mike, Sulley and Celia trapped. Boo stowed away and allows for a means to reactivate the door briefly, escaping with Celia back into the human world. Escaping the clutches of the pursuing villains, they end up leading them into another trap set up by the CDA to recapture the three.
In one of the film's early drafts, she was originally an 8-year-old boy named Raymond, who befriended Sulley (named Hob back then).
The fact that her full name is Mary Gibbs, with her nickname being "Boo", bares a strong similarity to that of the character, Arthur Radley, from Harper Lee's 1960 novel To Kill a Mockingbird. This is very plausible considering the fact that the two characters also share the same nickname of "Boo" although for two different reasons (Boo in Monsters Inc. for frequently exclaiming "Boo!" for fun, and the character from Mockingbird for being thought of as a scary individual).
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 because the filmmakers realized that the younger Boo was she could be more dependant on Sulley. 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 the youngest and only female child with siblings who was constantly picked on by her older brothers, 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 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.
In Toy Story 3, there is a girl in the Butterfly Room at Sunnyside Daycare who looks like a slightly older Boo. She is seen playing with a blue cat (Boo's nickname for Sulley is "Kitty") and says "Boo," seemingly reenacting a scene from Monsters, Inc. However, Lee Unkrich has confirmed that they are not the same character.
There is also speculation that Bonnie Anderson from Toy Story 3 could be an older Boo due to having Sulley's colors and print decorating her bathroom and having a similar appearance. However, those rumors are proven false due to the fact that Boo's real name is Mary Gibbs, not Bonnie.
Another older girl that looks more like Boo (including her hairstyle and pink shirt) appeared in the sequel Toy Story 4. She appeared In Bonnie's kindergarten class as well as the carnival.
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.
Originally, Boo was actually going to return as an adolescent in Monsters University (which, at the time, instead of a prequel, was to be a sequel film entitled Monsters Inc. 2: Lost in Scaridise).
In this film, Sulley and Mike, while preparing to celebrate her birthday one day, 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 bedroom, 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 does not even remember them anymore since children lose belief in monsters as they grow up, nor does she think they are 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 original film was just too heartwarming to be continued. However, the idea of a Monsters Inc. sequel has since been revived and put into fruition, but this time, in the form an upcoming web series entitled Monsters at Work set for Disney+.
Mary Gibbs, Boo's voice actress, was two years old at the time of filming for Monsters, Inc.. Since the crew found that Gibbs was unable to sit still in the recording booth, they followed her around the studio with a microphone to record her dialogue.
Some of Boo's cries and squeals are reused in Inside Out for a younger Riley Andersen. The credits list Mary Gibbs as an additional voice because of this.