At the start of the story, Jekyll is outgoing and healthy, but over the course of the events he slowly gets more ill/weak and becomes antisocial. From his confession we discover that this is because of him loosing control over his other side, Hyde. Since the effect of the potion has changed, Hyde becomes stronger while Jekyll grows weaker. As times goes on, Jekyll's mental and physical state continue to digress. Because of this, Jekyll regrets his actions.
Dr. Jekyll is a respected and relatively successful doctor at the beginning of the story. He is a prosperous man, known for being charitable and decent. However, as the story progresses, we learn that Dr. Jekyll has been engaging in strange behaviours that do not seem to match his character. Towards the end of the novel, we find that Dr. Jekyll has been engaging in experiments in order to separate his bad qualities from his positive ones. These experiments left him in the hand of Mr. Hyde, a personality that embodied only his negative qualities and thus became his "darker half."
