Dr. Henry Jekyll is a complicated character, though readers don't get a full picture of him until he explains his deeds and choices in the final chapter. Like all humans Henry Jekyll is, as he puts it, a "composite." His nature is both good and evil, civilized and primitive. Intrigued by this dual nature and wanting to experience the two separately, Jekyll finds a way to indulge his darker passions without it becoming known. Jekyll applies his knowledge of chemistry and invents a "tincture" that separates his good from his evil identity and even creates an entirely different body for each self. (Edward Hyde is his evil persona.) Above all Jekyll is almost classically arrogant. He believes he can reconstruct his own identity in order to break humanity's shared ethical rules and England's social norms, and without paying a price. Obviously he is wrong, and this novella is an account of his errors and how he pays for them.
Edward Hyde is the evil side of Dr. Jekyll's identity. He came into being when Jekyll invented a drug that would split his good and bad natures into two entities. Hyde even possesses a different body than does Jekyll. Hyde is younger than Jekyll but also hairier, as if he is more primitive. He is full of energy and is more evil than Jekyll's dark side had been. Everyone who sees Hyde finds him disturbing, but no one can name a single specific detail that makes him repellent. There's something about him that seems less evolved, like a caveman, but also something that seems purely evil. Hyde and Jekyll share a single memory. At first Jekyll must use the "tincture" he has created to transform himself into Jekyll, but after a while Jekyll finds himself transforming into Hyde spontaneously, first while he's sleeping and then while he's awake.
The interplay between Dr. Jekyll and Mr. Hyde is the engine driving this story. However, by the end, both of the two interwoven characters are dead. Mr. Utterson is the closest thing in the novella to a unifying consciousness or point-of-view character. Utterson is a lawyer and brings a lawyer's seriousness and logic to his interactions with other characters. Utterson is always aware of his responsibilities. At times, as when he's talking with Poole, he even warns people about this responsibility. However, Utterson's seriousness goes beyond his professional capacities. He is an austere man who actively practices discipline and resists temptations, even in his private life. This is most visible in the novella's opening paragraph. Utterson actively denies himself the things he enjoys, like drinking wine and going to the theater, precisely because he enjoys them. In this way he, more than Jekyll, is an anti-Hyde. Whereas Jekyll created Hyde to give his passions free rein, Utterson always holds a tight grip on his own actions and feelings.
Richard Enfield is one of several examples of Victorian respectability in this story. He is related to Utterson ("a distant kinsman"), and, though he is not nearly as austere as Utterson, he values their time together considerably. This kinship is literal, but it is also symbolic: these men align with virtue and civic duty, unlike Hyde. The two men take walks together every Sunday. It's on one of these that Enfield tells Utterson the "story of the door," which starts the narrative of Hyde (and Utterson's interest in him) in motion. Enfield's curiosity draws Utterson's attention to that mysterious door—and the reader is drawn in as well. Enfield is also Utterson's companion for the "incident at the window," when Jekyll must disappear because he's losing control to Hyde. Enfield functions as a witness to extraordinary events.
Dr. Lanyon is introduced in the second chapter when Utterson is searching for Mr. Hyde. He's important for several reasons. First, as Utterson notes, Lanyon is another of Jekyll's old friends. Their relationship deteriorates throughout the novella, which helps build dramatic tension. Second, Utterson is a lawyer, and his rejection of Hyde might be colored by that professional perspective. However, Lanyon is a doctor and so is qualified to evaluate Jekyll's project from a scientific perspective. Tellingly, he calls Jekyll's research "unscientific balderdash." Third, the story's sixth chapter focuses on Lanyon and the change in his health and relationship to Jekyll, though the meaning of that chapter is not revealed until the novella's ninth chapter, "Dr. Lanyon's Narrative." That later chapter reveals what Lanyon went through earlier: he saw Hyde change into Jekyll. This unnatural transformation shook Lanyon's mind and broke his health.
Poole is Jekyll's main servant and has been with the doctor for decades. In some sense, if Jekyll serves as Hyde's public face, Poole serves as Jekyll's public face. When Utterson first visits Jekyll in "Search for Mr. Hyde," he has to ask Poole if Jekyll is home. And Poole makes excuses for him. Once Utterson's relationship with Jekyll deteriorates sufficiently, he actually says he'd rather deal with Poole on Jekyll's doorstep than enter his friend's house. Poole's long service gives his testimony extra weight. His intimate knowledge of Henry Jekyll lets him speak with certainty in "The Last Night" and confirm for Utterson that it isn't Jekyll locked in the lab, but Hyde.