Duality is a major theme in all romantic novels. It is the idea that every person is made up of two halves, the good and the evil of man. In Dr. Jekyll and Mr. Hyde, Dr. Jekyll tries to split the two halves and bring out his evil side.
It appears in other romantic novels too, an example is Dracula. Dracula is the evil fighting the good that is represented by man. Dr. Jekyll manages to isolate and separate his evil side from the whole, creating in the process two very different people; Jekyll, who represents not pure good, but the whole of a person, and Hyde, who represents pure evil, and contains little, if any, of Jekyll in him. These two characters stand in stark contrast to one another through the novel, and Jekyll, in his narration of events, says are separate beings with separate motivations, but they are one being. Jekyll is aware of what Hyde is doing and even enjoys what he does at times.
It goes with the idea of Jack the Ripper, he is a proper and respected man during the day, but at night he is cold and is able to commit horrible actions with no remorse. Jekyll could even be called addicted to Hyde. When Hyde murders Sir Danvers Carew, Jekyll says that he is done with Hyde. Then soon after he is drawn back to Hyde, he struggles with the good and evil with in himself and he enjoys what Hyde does, but is scared of Hyde at the same time. In Dracula the theme of good versus evil takes on a religious quality with the two forces dividing into an actual combat between God and Satan. The brave band of men stands for and symbolizes God.
They are god's soldiers standing for all what is good and religious. There is definitely a feel of Christianity in the novel, which spills out in each of the climaxes. Dracula represents Satan, thereby standing for whatever is evil and bad. He hates goodness and tries to perpetuate evil in the world by biting women and making them vampires. The vampires are beastly and ugly. Even Lucy, when she becomes a vampire, is hated by this band of men, despite the fact they were in love with her before.
The vampires are brazen and the female vampires look on men as their prey for sexual release and their blood. The romantic novel is the never ending struggle of good and evil. The divided self and the war between God and Satan are two ways it is depicted. In both novels men fear the evil, but they are also drown to it; like Jekyll was to Hyde and the way Harker is in the beginning when he meets Dracula.