The Masked Mutant is a fictional character and antagonist from the children's book series of Goosebumps who was introduced in the 25th book, Attack of the Mutant and later its television adaption. He is an evil supervillain that has the power to turn himself into anything (except for liquids) in order to defeat his enemies, that of which includes the League of Good Guys, led by the super-fast Galloping Gazelle. But one day, the Masked Mutant grew bored with fighting his current enemies and wanted a new challenge— a new foe for his stories. So, he disguised himself as a girl named Libby Zacks; on a bus ride where he meets a boy named Bradley "Skipper" Matthews, his biggest fan.