Jekyll and Hyde
From Longman Dictionary of Contemporary EnglishJekyll and HydeJek‧yll and Hyde /ˌdʒekəl ənd ˈhaɪd/ noun [countable] CHANGE/BECOME DIFFERENTsomeone who is sometimes nice but at other times is unpleasantExamples from the CorpusJekyll and Hyde• As he spoke, his language grew rougher and rougher, as if a Jekyll and Hyde transformation were taking effect.Origin Jekyll and Hyde (1800-1900) 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