Oliver Haddo
Oliver Haddo is the title character of the novel and, like many characters in Maugham’s fiction, is loosely based on a real person. In this case, a notorious and infamous figure named Aleister Crowley who was termed the wickedest man in the world at one point due to his occult obsessions, strange behavior, and even more bizarre myths and legends which surrounded him. The term loosely especially applies in this case of basing a fictional character upon a real world analogue as the novel climaxes with the revelation that Haddo has been engaging in grotesque experimental abominations of the Dr. Moreau sort of genetic misbehavior.
Margaret Dauncey
Margaret is an attractive young art student studying in—where else—Paris. Haddo is immediately drawn to her sexually and sets about using his magical skills and dominant personality to seduce her. Whether in spite of or because she initially finds him utterly repulsive is not immediately clear. After she experience a surreal out-of-body experience while under his hypnotic powers, things change and she agrees to run off with him and get married, much to the dismay of Arthur Burdon.
Dr. Arthur Burdon
Arthur Burdon should very well be dismayed. In the first place, Haddo is physically a grotesquely obese caricature of a human being. More to the point: Margaret was engaged to marry him at the time she runs off with Haddo. After Margaret’s exit, her friend Susie Boyd makes plain her desire to step into the shoes vacated by Margaret, but Arthur cannot get over Margaret.
Dr. Porhoët
Dr. Porhoët is Burdon’s mentor in medicine, but is also the connective tissue to Haddo as a result of his scholarly study of the history and practices of the occult. Eventually, it is learned that Margaret has died while living with Haddo and based upon Arthur’s belief that Haddo has murdered her he calls upon Dr. Porhoët to engage in some black magic of his own to divine the truth. This action eventually leads to Haddo’s death and the revelation of his sinister secret experiments.
Margaret’s Terrier
Although his physical presence in the story is limited, the specter of Margaret’s little dog looms large. The key turning point in the narrative arises as a result of the dog’s instinctual recognition of bad medicine when he smells it enter a room. Urged as if by an unseen force of good to attack Haddo, the dog succeeds in landing some teeth into the magician’s bulky flesh which is a humiliation his fragile self-esteem is not prepared to handle. He viciously kicks the dog and Arthur quickly engages in retributive justice by thrashing Haddo to the ground in humiliation. From that point on, everything is about revenge for Haddo.
