From the Super Mario Wiki
Dr. Nerdnick is a spectacled, white-haired scientist who lives in Robo Land with his robot friend/aide Bunsen. He is a brilliant inventor who specializes mainly in building robots. Dr. Nerdnick also speaks only in a unintelligible language that only Bunsen seems to understand, thus forcing her to act as an interpretor between himself and other characters.
Dr. Nerdnick only ever appeared in The Super Mario Bros. Super Show! episode "Robo Koopa". Prior to the events of the episode, King Koopa came to Robo Land, where he somehow managed to get Dr. Nerdnick into building him a Robo Suit, which he used to become Robo Koopa, allowing him to terrorize the city. Whether Dr. Nerdnick was merely bribed or actually forced into building the robo suit is unknown. After becoming Robo Koopa, King Koopa cuffed Dr. Nerdnick to an operating table in his own workshop, due to the fact that he was the only person who was smart enough to devise a plan that could stop him.
Bunsen managed to escape from Robo Koopa, found Mario, Luigi, Princess Toadstool, and Toad in Robo Land, and brought them back to the workshop. With their help, Bunsen was able to rescue and escape with Dr. Nerdnick, but not with Princess Toadstool and Toad getting trapped in Robo Koopa's force fields.
After escaping from Robo Koopa, Dr. Nerdnick led Mario and Luigi to a junkyard, where he and Bunsen constructed the Plumbinator; a robo suit of their own that they could use to fight Robo Koopa with. After fighting against Robo Koopa, the Plumbinator was destroyed, and Robo Koopa succeeded in recapturing Dr. Nerdnick. With Bunsen acting as his interpretor, Dr. Nerdnick tricked Robo Koopa into thinking that his robo suit was going to run out of fuel unless he pushed the ejector seat button. Robo Koopa pushed the button, and was blasted out of the robo suit, thus ending his reign of terror.
Afterwards, Dr. Nerdnick concluded that Robo Koopa's suit was too dangerous to keep around. As a reward to Mario and Luigi for their help, he converted the old robo suit into a robo pizza oven for the two of them to use.