a person who is sometimes very pleasant (Jekyll) and sometimes very unpleasant (Hyde) or who leads two very separate lives He leads an almost Jekyll and Hyde existence—by day he's an accountant, by night he's a singer in a rock band. From the story by Robert Louis Stevenson, Dr. Jekyll and Mr. Hyde, in which Dr. Jekyll takes a drug which separates the good and bad sides of his personality into two characters. All the negative aspects go into the character of Mr. Hyde.
Check pronunciation: Jekyll and Hyde