Sherlock Holmes is the ever-observant, world-renowned detective of 221b Baker Street. For all his assumed genius and intuition he is virtually omniscient in these stories, and Holmes becomes more accessible in the context of his constant posturing and pretension.
Holmes lets down his guard and admits of a fragile ego. When challenged at the beginning of the book—Mortimer calls him the second best crime solver in Europe and Holmes lets down his guard and asks who could possibly be the first. By and large, however, Holmes' ego is kept in check by a constant dose of adulation from Watson. Holmes regularly announces some absurd and unsubstantiated conclusion only to mock Watson by revealing the most obvious of clues. In the end, Holmes toys with his associates (and particularly Watson) at least as much as he flouts his enemies, equivocating, misleading, and making fools out of them only to up his own crime-solving cachet.
The good doctor plays the sidekick to Holmes' self-obsessed hero figure. Watson is a lowly apprentice and live-in friend, who spends most of the book trying to solve a difficult case in his master's stead. Always on hand to stroke Holmes' ego, Watson is nonetheless intent on proving his own mettle by applying Holmes' techniques.
Watson's never-ending adulation, which is presumably meant to mirror our own understanding of the legendary detective, comes through most forcefully at the end of the novel, when Holmes arrives at Devonshire. Holmes announces that he meant for Watson to think he was in London, and a pouty Watson reacts: "Then you use me, and yet you do not trust me!" Codependent throughout, Holmes and Watson fill each other's needs. Watson provides Holmes with an ego boost, and Holmes needs Watson's eyes and ears to inconspicuously gather clues. Watson is awestruck by Holmes' power of observation, and Watson feels more powerful by association.
Intended to incarnate ill will and malice, Stapleton is conflated at various points with the lecherous libertine Hugo, whom he resembles. Stapleton is a black-hearted, violent villain hidden beneath a benign, bookish surface.
If Hugo operates as a kind of Doppelganger for his entomologist heir, then the convict offers an interesting parallel as well. Serving mainly as a red herring in the mysterious death of Sir Charles Baskerville, the convict also operates as a foil for the real culprit, Stapleton. Personifying "peculiar ferocity," "wonton brutality," and even dubious sanity, the convict is shown to be a pathetic, animalistic figure on whom the detectives ultimately take pity. Not so with Stapleton, a man with a "murderous heart," and a wolf in sheep's clothing.
Stapleton is a worthy adversary because of his birthright. If the convict is a simple murderer, he is also simply born, related by blood to the Baskerville's domestic help. Thus, the convict is part of a lower class than Holmes, and therefore is not a worthy adversary. Stapleton, however, is an intellectual, and when his evil side comes out, his hidden nobility comes out as well. Once Holmes is handling an educated and noble rival, he begins to take things much more seriously. In this sense, Stapleton's character adds to the strong classist themes imbedded in this book.