Jimmy is a small cockroach with a humongous appetite. The one thing he dislikes the most is others trying to kill or squash him. He appears to be quite harmless and enjoys being fed by his owner Reuben.
When Gantu notices Jimmy, he makes a vain attempt to kill the bug, much to Reuben's horror. Reuben then reveals Jimmy to be his pet and ignores Gantu's demands to exterminate the pest. Later, when Gantu steals Experiment 029 (Checkers) and takes over as king, Reuben and Stitch rally an army of other experiments to rebel against Gantu. Reuben even uses Jimmy to help out, and Gantu is successfully overthrown. Reuben then congratulates Jimmy on a job well done, while Gantu is given the punitive duty of cleaning the ship's bathroom under Hämsterviel's orders.