a Jekyll and Hyde

someone whose personality has two different parts, one very nice and the other very unpleasant

Usage notes: This phrase comes from the main character in the book The Strange Case of Dr Jekyll and Mr Hyde by Robert Louis Stevenson.

You can't depend on him to be friendly - he's a Jekyll and Hyde.

Name	Pronounced	Usage
Pompilius	-	Ancient Roman
Gwandoya	-	Eastern African, Ganda
Bertie	BUR-tee	English
Adeodatus	-	Late Roman
Felix	FE-liks (German, Swedish), FAY-liks (Dutch), FEE-liks (English)	German, Dutch, Swedish, Norwegian, Danish, English, Romanian, Ancient Roman, Biblical, Biblical Lati
Darach	-	Irish