The relationship between Dr. Jekyll and Mr. Hyde is integral to the theme of duality. When we discover that they are the same person—that is, that Dr. Jekyll transforms into Mr. Hyde—we understand the point that Robert Louis Stevenson is trying to make about the duality within us.
The two personas have different appearances and different personalities. They are so different that they are essential opposites of each other. Their opposite nature develops the idea of having two opposite sides to oneself.
Hence, although I had now two characters as well as two appearances, one was wholly evil, and the other was still the old Henry Jekyll, that incongruous compound of whose reformation and improvement I had already learned to despair.
Dr. Jekyll recognizes Hyde as an evil being. For the most part, Jekyll hates Hyde.
I, who sicken and freeze at the mere thought of him, when I recall the abjection and passion of this attachment, and when I know how he fears my power to cut him off by suicide, I find it in my heart to pity him.
This quote is interesting because Jekyll admits that he pities Hyde. It also shows that Hyde recognizes the power Jekyll has by being able to commit suicide, therefore killing Hyde as well.
Henry Jekyll stood at times aghast before the acts of Edward Hyde, but the situation was apart from ordinary laws, and insidiously relaxed the grasp of conscience. It was Hyde, after all, and Hyde alone, that was guilty. Jekyll was no worse; he woke again to his good qualities seemingly unimpaired; he would even make haste, where it was possible, to undo the evil done by Hyde. And thus his conscience slumbered.
Jekyll separates himself from Hyde and puts the blame all on Hyde. He then tries to "undo the evil."