in reference to opposite aspects of a person's character is a reference to Robert Louis Stevenson's story, "The Strange Case of Dr. Jekyll and Mr. Hyde," published in 1886. Jekyll, the surname of the respectful and benevolent man, is of Breton origin and was originally a personal name. Hyde in reference to the dark, opposite side of one's personality is from 1887.
"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]