Mr. Toggle is the main antagonist in Goosebumps book Piano Lessons Can Be Murder.
He is an inventor who worked for the choice of piano, but he has a great obsession with hands. He is also a serial killer, and also a genius in robotics, coupled with his obsession with hands. He is also a serial killer, and also a genius in robotics, coupled with his obsession with creating beautiful music that he used these abilities to create his own hands.
However, it is later revealed that he stole the hands of the boys he used in music, and in his robots, since he was unable to create robots with hands. In addition, he is the creator of Jerry's piano teacher, Dr. Schreek who is living robot. And right afterwards at the end of the book, the old ghosts of the boys who were killed by Mr. Toggle, chase him at school. Then in the end, Mr. Toggle is punished by being forced to play the piano for eternity.