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, and 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, so 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.
Although he initially doesn’t believe Cooper about the ghost dogs, Mickey changes his mind when Cooper and Fergie enter his bedroom through the walls as the ghost dogs and catch him in his underwear, and begin growling at him. Unfortunately for Cooper, Mickey doesn’t realize it’s him. Later, Cooper and Fergie overhear Mickey attempt to tell his and Cooper’s parents about the ghost dogs he saw, saying that they “walked right through” his bedroom wall, and that they weren’t normal.
However, Mr. and Mrs. Holmes don’t believe him, and Mr. Holmes snaps at Mickey to save his jokes for Cooper.
Differences in the TV episode
In the TV adaptation of the book, Mickey is pushed into the Changing Tree and switches bodies with a chipmunk, rather than Cooper and Fergie turning into chipmunks like in the book. He calls out for help after Cooper and Fergie leave.
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.