in reference to opposite aspects of a person's character, from Robert Louis Stevenson's story, "The Strange Case of Dr. Jekyll and Mr. Hyde," published 1886. The surname Jekyll is of Breton origin and was originally a personal name.
"Though so profound a double-dealer, I was in no sense a hypocrite. Both sides of me were in dead earnest; I was no more myself when I laid aside restraint and plunged in shame, than when I labored, in the eye of day, at the furtherance of knowledge or the relief of sorrow and suffering." [Robert Louis Stevenson, "The Strange Case of Dr. Jekyll and Mr. Hyde," 1886]