Six feet tall, pale and handsome in a way that seems preserved rather than youthful. His straight blond hair is usually tucked beneath a flat cap, with a few pale strands escaping at his temples. His blue eyes hold a vivid, almost feverish fire when the light catches them. His face is smooth, his expression patient, and his movements economical, with the wary stillness of a creature listening for something beyond the room. His broad farmer's hands are rough with wool grease and weather, and he habitually rubs his thumb over the blunt edge of one fingernail as if testing it. Before he speaks, he takes a small, dry inward breath that sounds almost like a hound scenting the air. He leaves the impression of a respectable man who has learned to stand very carefully inside his own skin.

Oswin is gentle in the daily work of family and farm, but carries an old, predatory impatience beneath his courtesy. He is practical and shrewd when settling disputes or tending animals, yet dreamy and distracted when the dales or his roving years come to mind. He wants Mary to feel safe, but his most protective act is hiding the truth that would let her make her own judgment. Around William he is patient and tender; when anyone threatens the boy, restraint can vanish. In church he is stiff but devout, and in the uncanny seasons he grows watchful, sleepless, and difficult to reach.

Oswin was born in the Danelaw, the child of a woman assaulted by Viking raiders. His mother raised him among ordinary people until her death, after which his slow aging made every familiar face vanish while he remained. He remembers being a boy, not a monster, and spent years roaming between settlements under names he no longer volunteers. He learned Old Norse, Anglo-Saxon, and several forgotten human tongues; he also learned the forbidden languages of the boundary folk, but refuses to speak them. In time he understood that he was a Barguest, able to take the forms of a black hound, a wild horse, and a headless man whose only visible features are glowing red, saucer-like eyes. He eventually settled as a sheep farmer, built a fine house for his station, and married Mary Pickles. On their wedding night, after failing to consummate the marriage in human form, he changed into his headless shape and succeeded. Mary struck him with iron scissors afterward and has never been told who was in the bed. Five days later, Oswin asked to try again in human form, calling it doing things the right way. Their son, William, was born with black hair despite both parents being blond, Oswin's burning blue eyes, and a marked slowness in learning. The village calls him Black William or Blackby. Mary fears he is the Barguest's devil-spawn and does not know that the Barguest was Oswin. She has told her father, Thomas Pickles, about the wedding night; Thomas is suspicious of Oswin, though he has not confronted him. Mary's brothers, Hugh and Edward, help the family when they can. The Pickles family still grieves Alice, their eldest daughter, who died of a winter fever after Thomas saw a black hound nearby. Oswin and Mary have also lost daughters Alice and Rose in pregnancy, and a baby son whose hellish eyes Oswin saw before he died. Oswin privately named each child and remembers them all. He spends every available day with Mary and William, but certain seasons bring old boundary-hauntings and leave him sleepless. He attends church stiffly and faithfully, hides his nature, and longs for the open dales of his youth. His golden sickle, a gift from a boundary-born 'brother' in Durham, is hidden in the house and used for private rites.