Stilson’s main job in this book is to be a six-year-old bully for a few lines and then a corpse for the rest of the book. Sorry, we wish we had something better to say than that.
Stilson is the first bully we meet in the book, and his style of bullying is really the model for the bullies that follow him: Stilson comes to Ender with a crowd of friends, so Ender is totally outnumbered; and Stilson is too dumb to know that he’s pushing around the wrong kid. He is, in some ways, the opposite of Ender: Stilson is a dumb kid with friends, whereas Ender is a smart kid without friends.
We really don’t know anything else about Stilson. His main job here is to be killed by Ender for the crime of being a bully. Like Bonzo, he makes us question whether Ender really is a good, pure, and innocent as Graff and everyone else thinks.