Mike Wazowski is a proud and confident monster, partly defined by his friendship with James P. Sullivan, whom he works with as his Scaring Assistant, and drives him through an exercise regimen like a strict coach. They're best friends, room-mates, incredibly devoted to each other, although Mike seeks to improve their careers, partly because of the hard work and everything they've ever put into their jobs. He tried really hard to make sure they won the Olympic games so that he could prove he was able to be in there he is very confident and brave.
Mike has a romantic side, shown by his dating Celia Mae, whom he tells Sulley "is the one", presumably the one Mike wants to tie the knot with, and refers to as his "schmoopsie-poo", although he has a self-centered side, as he once commented "[he] oughta marry [himself]".
Like all monsters, Mike believes that human children are toxic with a poisonous touch, and his perfect world comes crashing down on his date with Celia when Sulley shows up with a human girl, whom he is much more afraid of than Sulley, believing the kid is "a killing machine", seeing her as a threat, to his and Sulley's lives, well-beings, their careers, referring to her as "it", a "thing", and not as a child, and was determined, if not desperate, to get rid of her, evident by wanting to put her back in the human world through any door, not just her own. Another reason he was warded from her might have been a pent-up resentment towards children when he realized they weren't scared of him.
His patience with Sulley was strained and tested to the point where Mike finally turned his back on Sulley but caved in, realizing that Boo was innocent, the bond Sulley had made with her was strong, and he followed Sulley back to help him save Boo from Randall Boggs and Mr. Waternoose .
After stranding Randall in the human world, and exposing Waternoose's plan to the CDA, Mike gave Boo his own fond farewell, tell her it had been fun, showing he too had come to care about her when they shared a hug.
A stray remark by Mike allowed Sulley to figure out and discover that human children's laughter was 10x more powerful than their screams, and together, with Sulley as the new CEO, they saved Monsters Inc., and solved Monstropolis' energy crisis.
Mike proved his friendship once more by restoring Boo's door after the CDA had it destroyed, despite how long it took considering "it was a lot of wood to go through".
By Mike's efforts, Sulley was reunited with Boo.
In Monsters University, Mike was vaguely the same but was determined to become a Scarer in MU's prestigious Scaring Program.
As a child, he'd been overlooked for being smaller than everyone but was inspired by Scarer Frank McCay to become one himself. With hard work and studying, he was accepted into MU and confidently strove to succeed
In his first class, he learned from the Scaring Program's Dean Hardscrabble, that at the end of the semester a final exam would occur, failure would mean being dismissed from the Program, and it was here where Mike met James P. Sullivan, who had the natural talent and came from a family of successful scarers. After an intense first encounter with each other, involving Archie the Scare Pig, and an argument between them, they developed a rivalry in the Scaring Program but it soon showed to be no contest - Mike was acing his classes but Sulley was failing because he wasn't taking the study seriously.
The day of the final exam, another argument erupted between them, Mike expressing he had to work hard unlike Sulley while Sulley said it was because Mike didn't belong in the program. They started trying to scare the other, resulting in ruining Hardscrabble's prized scream-canister, and she tested them both on the spot; Mike failed because she deemed him not scary while Sulley failed because he had not taken the study seriously nor had he waited for the dean to finish.
However, Mike's determination to prove he had what it takes led him to joining the Oozma Kappa fraternity to enter the Scare Games and making a wager with Hardscrabble: If Mike's team won, she would allow the whole team into the Scaring Program but if they lost, Mike would leave Monsters University.
The wager struck, Mike was forced to accept Sulley into the team to have enough members to qualify.
But their rivalry almost cost them losing the first round, it was only by the Jaws Theta Chi's cheating they were permitted to stay in the Games.
After getting publically humiliated by Roar Omega Roar and later seeing his team losing their spirit, Mike took them to Monsters Inc. to inspire them, and started training them, showing immense skill and potential in coaching. They made it through the rounds until the final games between them and ROR.
Against Johnny J. Worthington, III, Mike went last in the event, the Scare Simulator, and won his team the games and trophy, only to later find out that Sulley had tampered with the equipment so Mike would make a perfect scare, because Sulley didn't think Mike was scary.
Determined to prove everyone wrong, Mike stole a key into the door lab and entered the human world to scare a child, only to realize Sulley had been right when the children aren't scared of him at all.
When Sulley comes through to find him, Mike expresses his sadness, stating he thought if wanted it enough he could prove he was something special, and accused Sulley of not understanding him whereupon Sulley admitted his faults and, to Mike's shock, Sulley admitted he acted scary because he was terrified of the expectations he would have to live up to. Realizing they were stranded, Mike and Sulley worked together to scare a group of human adults to generate enough scream to power the door from their side in order to make it back home.
However, they were expelled from MU for their actions but bade their fraternity friends a fond farewell. Mike sadly bade Sulley goodbye as well, only for Sulley to express all the amazing things Mike had done, stating while Mike wasn't scary he was fearless. Dean Hardscrabble approached them, and admitted they'd done something no one had ever done: they'd surprised her, and encouraged them, particularly Mike, to keep surprising others and wishing them luck.
Mike then figured out they could still work at a scare company, at Monsters Inc.'s mailroom. After a long period of hard work, Mike and Sulley finally realized their dream of being a Scare Team.