Mike Wazowski is a proud and confident monster, partly defined by his friendship withJames 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 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 .