Je·kyll and Hyde

/ˈʤɛkələnˈhaıd/ noun [singular]

: someone who is sometimes good and pleasant and sometimes very rude or bad

He's a real Jekyll and Hyde who can become violent without warning.

his Jekyll and Hyde tendencies

◇ This term comes from “The Strange Case of Dr. Jekyll and Mr. Hyde,” a story by Robert Louis Stevenson. In the story, Dr. Jekyll is a scientist who creates a liquid that changes him from a good man to the evil Mr. Hyde when he drinks it.