Robby was locked in a cage in Carnival World, with a sign that reads PLEASE DO NOT FEED THE WEIRDO. However, Jordan and Karla Keppler feed him, which results in him transforming into a furry, green, vomit-spewing monster. His orange vomit spews everywhere, and everyone leaves the carnival. It would be very difficult to capture Robby, since he can shape-shift into anyone. Robby escapes the carnival and Manny Ferber, the owner of Carnival World, sets off to capture him.
This is followed by several encounters with Robby, all resulting in him escaping. Jordan then figures out how to capture the monster. His plan is to use the food from the upcoming school picnic as bait. When Robby appears, Ferber, hidden behind the bushes, would throw a strong net over him.
The next day, Jordan and his classmates attend the picnic. He starts to worry because Robby wasn't anywhere in sight. Jordan's teacher suggests trying out some rides. They start at the Tunnel of Fear, where Jordan finds Robby. Robby disguises himself as Jordan, convincing everybody that he is in fact the real Jordan. Mr. Ferber throws the net over the real Jordan instead of Robby and locks him in a cage, assuming he was actually Robby in disguise.
Robby's monster form resembles a giant bear with green fur and giant, green paws. The book's cover depicts Robby as having yellow eyes. Robby can shape-shift, granting him the ability to disguise as anyone. He looks like a regular brown-haired boy in his "Robby" form.
Robby is always pleading to be fed, and would turn into a vomit-spewing monster if anyone relents and feeds him. His timid demeanor and attitude often helps him get a bite every once in a while. Robby is also a trickster who is known to lie just to get a free meal.