be (a) Jekyll and Hyde

To have a personality that swings drastically between pleasant and intolerable. A reference to the main character of the book Strange Case of Dr. Jekyll and Mr. Hyde, by Robert Louis Stevenson. Ever since Kyle became a teenager, he's turned into a real Jekyll and Hyde, and it's impossible to anticipate his mood at any given time. Our coach was always Jekyll and Hyde on the field. If we were winning, he was your best friend, but if we were losing, he'd treat you like dirt. Before he went to rehab, Stan used to be Jekyll and Hyde every time he drank too much.