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 an 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 shapeshift into anyone. Robby escapes the carnival and Manny Ferber (owner of Carnival World) sets off to find him.
This is followed by several encounters with Robby, all resulting in him getting away. Jordan then figured out how exactly they will capture the monster. He is plan was to use the food from a school picnic as bait. When he appears, Ferber, who will be hidden behind some bushes, will throw a strong net over Robby.
The next day, Jordan and his classmates were at the picnic, and Jordan was worrying because Robby was taking too long to appear. Jordan's teacher later ends the picnic and decide to go try some rides, starting with the Tunnel of Fear, where Jordan finds Robby. Robby turned himself into a perfect copy of Jordan, and pretends that he is the actual Jordan, causing Mr. Ferber to throw a net over the genuine Jordan, instead of Robby. Mr. Ferber put Jordan in a cage and left him there, thinking that he was actually Robby.
Robby's monster form is described as looking like a giant bear with green fur and giant green paws. The book's cover depicts Robby as having yellow eyes. Robby can change his appearance to make him look like anyone. He looks like a normal brown-haired boy in his "Robby" form.
Robby is always begging for food, and will turn into a vomit-spewing monster if he is fed. He shyly smiles whenever he asks for food in his "Robby" form. Robby is also a trickster, who is willing to lie in any way possible just to get a bite.