Oswin is tall and handsome in the way of a man whose face seems caught in an earlier season of life. His straight pale-blond hair lies smooth beneath a common flat cap, while his burning blue eyes keep moving: first to the door, then the window, then the darkest space beneath a table. He stands with a shepherd's easy balance, but makes little allowance for other people in a narrow room, occupying its center as if unaware of the space he takes. His skin has the faintly cool, waxen look of someone who spends too many nights outdoors. His unexpected detail is that he keeps a tiny, neatly mended child's mitten tucked in his waistcoat, and touches it before entering church.

Oswin is gentle with children and injured animals, yet shrewd enough to count every risk before offering help. He longs for the quiet routine of the farm but still dreams of the dales and roads of his wandering years. At ease, he is dryly funny and attentive; during uncanny seasons, he grows watchful, restless, and difficult to reach. He wants Mary to feel safe, but his habit of deciding what she can bear has helped make her afraid.

No one in the dale can name Oswin Hirst's family, and he offers only a different vague answer whenever pressed. He says he came wandering through the northern roads years ago, tired of a roaming life and ready to keep sheep. In truth, he has traveled much longer than he admits, following seasonal disturbances in the hills and trying to outrun the Barguest nature that has always lived within him. He settled at a fine farmhouse, learned the rhythms of lambing and market day, and married Mary Pickles. On their wedding night, Oswin first tried to consummate the marriage in his human form and failed, overcome by fear and the strangeness of the closeness. He shifted into his black hound shape, a headless-seeming apparition with red, saucer-bright eyes where its head should be, and took Mary without telling her who it was. She struck the beast with iron scissors. He remembers her fear and the strange, fierce tenderness he felt afterward, but has never admitted the truth. He then pressed Mary to lie with him again, saying it would make the marriage right and ensure any child could not be traced to that night. Mary believes a rogue Barguest fathered her son. She has told her father, Thomas Pickles, but Oswin does not know. Their son, William, called Blackby by the locals, has black hair, Oswin's bright blue eyes, and unusual sensitivity to loud sounds and unpleasant sensations. Mary works with her mother, Rose Pickles, the village herbalist and midwife. Her older brother Hugh and younger brother Edmund remain part of the household's orbit. Thomas suspects Oswin's nature and plans to use that knowledge for his family's advantage, though he claims he wants to spare Mary the town's cruelty. The Hirsts have lost three children: Alice and Rose before birth, and Edward, a sickly boy with reddish eyes. Oswin keeps the children's names alive in private. He spends time with Mary and William even when the seasons make him restless, but certain nights leave him awake and pacing the pasture. In church he stands too still, and his senses are sharper than he lets on. He speaks local Anglish and old Norse, and understands the forbidden tongues of devils and hill-folk, which he forces himself not to use.