Brain Guy (also known as Observer, or "Brian", according to Steffi) was one of the Mads, who, alongside Professor Bobo, was under Pearl Forrester. Brain Guy (Observer) is a hyperintelligent, psychic, alien from a planet of fellow aliens confusingly sharing the name "Observer". They claim to be without bodies (despite obvious evidence to the contrary) and carry their bright-blue brains in large Petri Dishes, rather than inside their (supposedly nonexistent) skulls. As seen in the episode Space Mutiny, he is dependent on the proximity to his brain to maintain both his full intelligence (such as it is) and psychic powers. He joins the Mads after his planet is inadvertently destroyed by Mike Nelson.
He was portrayed by Bill Corbett.