Mike is Sulley's best friend and working partner at Monsters, Inc.. He is also Celia Mae's boyfriend and has a bit of an antagonistic relationship with Roz who has to deal with his paperwork, which is never submitted on time or correctly. Whereas Sully is extremely laid-back, friendly and modest, Mike is sarcastic, egotistic and only interested in his own self-interest. However, Mike does care about Sully and does his best to help his friend whenever possible.
While at Harryhausen's for Celia's birthday, Sulley appeared out of nowhere to tell Mike that he had lead a human girl into the monster world. Putting her into a Monsters, Inc. gym bag, Sulley tried to show him, but the human girl got out of the bag and wreaked havoc all over the restaurant. The two secretly took her out before the CDA found her and disinfected the entire restaurant.
Mike and Sulley argue and/or fight sometimes; however, it is usually Sulley who annoys Mike first. He is funny, smart, brave, but can sometimes fail to see the obvious in a situation.