Mary Beth was a famous cosmetics mogul. She was also an immortal monster who had been alive since the dawn of time -- a feat which she accomplished by draining and drinking the distilled essences of superheroes. She sent thousands to their doom, and enjoys Gilligan's Island greatly. She was in love with Cosgrove, whom she called her "Chunky Lumpkins," and wished to make him immortal so that they could live together forever, but he refused to let her drink Freakazoid's juices, and she perished.
Mary Beth was voiced by Tress MacNeille.