Jek ‧ yll and Hyde /ˌdʒekəl ənd ˈhaɪd, ˌdʒekɪl ənd ˈhaɪd/ BrE AmE noun [countable]
[ Date: 1800-1900 ; Origin: The Strange Case of Dr Jekyll and Mr Hyde , a story (1886) by Robert Louis Stevenson about a man with a good character and an evil character ]
someone who is sometimes nice but at other times is unpleasant