I Was Quite The Looker: Not to us (Unless you're into that sorta thing) but if you pay close attention, you’ll notice that back in his college days he was a lot leaner, had brighter, shiner fur, and more of it on top of his head.
Inadequate Inheritor/"Well Done, Son" Guy: He mentions to Mike that despite his arrogance, he is quite insecure due to the pressure the Sullivan family puts on him, as Sulley's father was a famous scarer (hence doing anything he can to get back into the Scaring Program).
Ink-Suit Actor: He's really just a hairy John Goodman, well, a hairier John Goodman.
Jerkass: In Monsters University, he's largely inattentive, apathetic, and outright rude to many of his peers.
Earn Your Happy Ending: Mike and Sulley get kicked off the scare program at Monsters University, take 6 misfits and turn them scarer material just for the chance to get back on. They end up expelled. Undeterred they decide to work their way up together at the ranks at Monsters Inc, from the mail-room up, until they become scarers. They then become the greatest scaring team on record. Only to uncover corruption at their workplace and inadvertently close down the business they worked so hard for. Fortunately they discovered a new means of power along the way. Fittingly, having lost his dream of being a scarer, it's Mike who now gets to take the lead and become one of the greats in his business.
Growing Up Sucks: Sacrificed his life-long dream as an on-field Scarer. Even though he works his way into being treated like an equal to on-field Scarers, it doesn't make his past pains any less significant.
Also averted, as he and Sulley climb the ranks at Monsters Inc. until they become a full-fledged scare team. Mike's hard work ultimately brings him to the position as a scare coach rather than a scarer. Not only that but one of the top scarers in the company. Heck even before then, as Sulley and he were working their way up the company. They would in the top in a lot of simple fields such as Mail Clerk, Janitorial Work and Cafeteria Workers. Before ultimately landing a job as scarers, no doubt thanks to his coaching.
Horrifying Hero: Subverted, Mike is extremely knowledgable, but isn't very scary the same way Sully can be. Even when he attempted to do so on his own in the second film, none of the kids were scared of him, and in fact found him kind of adorable.
Ink-Suit Actor: Even as an adorably inhuman monster, Mike could not be more Billy Crystal.
Sidekick: Interestingly enough, Mike sees Sulley as hissidekick. In Monsters University, this is explained by showing Sulley as being very dependent on Mike's intellect and knowledge.
The Strategist: Thanks to being book smart, Mike formulates plans and tactics that help out greatly during the second film, such as whipping Oozma Kappa into a fairly competent scaring team, and later when he and Sully get trapped in the human world, and he devises a plan to scare the rangers so that they can power the door from the human side. It also has the bonus of surprising Dean Hardscrabble, which had never been done before.
Stepford Smiler/Stepford Snarker: On his school field trip. He's acting really chipper and occasionally snarks about, but both when he ends up without a field trip buddy and when a classmate tells him he does not belong on a scare floor, it's clear how disheartened he is. Shades of this character trait are also present when he is older, especially in his reactions to Randall's treatment of him.
Took a Level in Kindness: Joins Oozma Kappa for his own motives in order to revive his chances in the Scaring Program. After barely passing the first competition, he holds Oozma Kappa to a rather conforming (ineffective) method in the library challenge. But when he sees that the Oozmas have competence of their own, he becomes more appreciative of them and more accommodating to their special abilities.
Tragic Dream: Worked from a very young age to become a scarer. Nobody truly believed in him and in the end it didn't work out, though he did become Sulley's best friend and discovers a true greatness in serving as scare coach along the way. Subverted when he turns out to have quite the talent for making children laugh and so he does become a great in his field of expertise after all. Albeit in a very different way than he'd thought.
Badass Adorable: She overcomes her fear of Randall at the very last second when she sees him about to give Sulley a Disney Villain Death, beating the crap out of him with her little fists and the help of a baseball bat.
Berserk Button: She has one. And Randall figured that out MUCH TOO LATE! Trying to kill Sulley in front of Boo will result in...this. Go to 7:22.
Berserker Tears: She actually cries just as much a kid her age would, but in the monster world, any loud noise from her can cause a blackout.
Method Acting / Throw It In: Initially during recording, they tried having Mary in a sound booth and encourage her to say some lines. After realizing how uncomfortable she was (and the challenge of trying to get a 3 year old to focus for long periods of time), they ended up with a rather amusing solution to the problem. They let Mary wander the Pixar building and just babble whatever she wanted to say while two sound guys followed her around with a microphone.
Adorkable: Not in the original movie, mind you, no way no how. We're talking about Monsters University. D'aww, look at those twiddling fingers and that cheeky little smile...
Always Someone Better: No matter how much effort he put into it, he'd never become a better scarer than Sulley, a fact Waternoose acknowledged to Randall even after the two of them banished Sulley for the sake of their Evil Plan.
Bitch in Sheep's Clothing: Despite being Mike's roommate in college, he was something of a popularity seeker who joins up with ROR and participated in the prank to discredit his old friend's team, although peer pressure might have been a factor.
Blind Without 'Em: Well, not necessarily blind, but after he abandons his glasses due to Mike suggesting he do so in order to make the most out of his camouflage ability, he keeps his eyes constantly squinted in order to see. By the time Monster's Inc comes around, he's still squinting.
Card-Carrying Villain: He apparently knew what he was doing was evil, as he told Fungus not to fraternize with victims of his evil plot.
Face-Heel Turn: He used to be a friend of Mike's in Monsters University, but decided to partner with Roar Omega Roar instead and swore revenge on Sullivan for accidentally ruining his scare simulation.
Freudian Excuse: Averted. He starts out genuinely nice and kind, but nothing awful happens to him in the slightest, and his Start of Darkness was still his own choice. No tragic backstory for you...
Social Climber: In Monsters University he was one of these, ditching Mike at the first opportunity to hang with the cool teens.
Sore Loser: In Monsters University, his loss to Sulley in the final event of the Scare Games gets him mocked in front of the entire school (he got the lowest score of his teammates), lost him all his friends, and served as a catalyst for his resentment of him in Monsters Inc..
Start of Darkness: In Monsters University, he slowly turned from a nervous character who was close friends to Mike Wazowski to a resentful foe to Mike and Sulley.
Even Evil Has Standards: It's hinted that he merely tolerates Randall and views his tactics as a Necessary Evil in his plan to bring Monsters, Inc. back to profitability. He even bluntly tells Randall that he's half as scary as Sulley at best.
First Name Basis: The only one in the movie to address Sully by his first name James.
The Mentor: Implied to be such for Sulley. Waternoose is sincerely despondent when he has to get rid of him, and in the climax Sulley quite truly hopes he can convince him not to go through with the plan.
From DVD bonus material: "The pink copy [of your carbonless copy paper scare receipt] goes to accounts receivable. The fuchsia copy goes to purchasing. Ivory goes to your immediate supervisor. And I! Get! The goldenrod! Goldenrod!"
Reasonable Authority Figure: In spite of being the head of an organization that is ridiculously paranoid in dealing with humans and hearing the many false horror stories about them, Roz's first inclination upon meeting Boo is to send her safely back to the human world.
Reverse Mole: It turns out that she is the Agent #001 of the CDA.
Jerk Jock: Subverted. He was part of the Jaws Theta Chi, but during the scene of Oozma Kappa's humiliation at the ROR house, he is the only member of his frat who looks distraught at what's going on.Word of God says, "he's a good kid in the wrong frat."