* Peter: The protagonist of the story, a 10-year-old boy who is forced to share his room with his grandfather when his grandfather moves in.
* Ed (Grandpa): Peter's grandfather, a mischievous and competitive man who loves to play pranks and win.
* Sally: Peter's mother and Ed's daughter.
* Arthur: Peter's father and Sally's husband.
While there are other characters in the story, these four are the most central and drive the plot forward.