The story "Strange Case of Dr. Jekyll and Mr. Hyde" can be said to represent a case of dissociative identity disorder, which is sometimes colloquially referred to as "split personality." Contrary to what some people may think, this story is not a metaphor for bipolar disorder, which typically includes fewer mood swings than the disorder's name indicates.
In the story, the two characters, Dr. Jekyll and Mr. Hyde, have dramatically different personalities and moral compasses, with the doctor tending toward mild-mannered behavior and Mr. Hyde presenting some truly evil proclivities. These dramatically different personalities are shown eventually to exist within one body. This is a potential symptom of dissociative identity disorder.