The invisible robot was a prototype robot built by Grimer for Doctor Robotnik. As the name suggests, it was fitted with a cloaking device that rendered it completely invisible to the naked eye. It was sent to terrorise the Tropical Jungle Zone. It initially managed to knock down Sonic, although Sonic was immediately on his feet again. The robot was defeated when Sonic first drenched it with water and then covered it with vine leaves, making it visible and obscuring its own visual circuits. Then he hit it with a Spin Attack, damaging the cloaking device, and knocked it into a lake so that the exposed wires made it short circuit.
Despite the defeat, Robotnik nevertheless intended to build a whole army of invisible robots, until he learned that Grimer had accidentally made the plans for the technology invisible as well.