Mickey likes to scare his easy-to-scare younger brother, Cooper. He enters the story by hiding under Cooper's bed and jumping out at him wearing a mask.
He gives his brother a hard time for being scared all the time. He tells Cooper when school starts to not tell anyone they are related.
Mickey goes to great lengths to prank Cooper. While Cooper and Fergie are in the woods, Mickey rips up his clothes, applies fake blood, and falls on the ground in front of them to make them think he was attacked by dogs.
He is always one step ahead of Cooper. He saw Fergie enter their house with a fake rat and expected to be pranked by Cooper and Fergie. He stuffed towels and blankets in his bed to make it look like he was sleeping and hid in his closet before Cooper and Fergie came in to scare him. He jumps out and scares them.
Mickey does not believe Cooper about the ghost dogs. He finally believes when Cooper and Fergie enter his bedroom through the walls and catch him in his underwear. Unfortunately for Cooper, Mickey does not realize it's him.
Differences in the TV episode
In the TV adaptation of the book, Mickey enters the Changing Room and switches body with a chipmunk rather than Cooper and Fergie like in the book. He calls out for help while Cooper and Fergie leave with Mickey and Cooper's parents.
Mickey is three years older than Cooper and a lot bigger. No other details are given about his appearance.
Mickey is a rather cruel older brother, often taking great joy in scaring and tormenting Cooper. He's also rather devious in doing so. This includes having Fergie trick Cooper into believing the story of the ghost dogs and pretending to be hurt later in the book to scare Cooper some more. Not to mention the more extreme cases like grabbing Cooper by the throat and pretending to strangle him.