Oswin is tall and handsome in the weathered, plainspoken way of a prosperous farmer: a straight nose, a youthful face that has barely changed over the years, and pale blond hair that lies smooth even in damp weather. His blue eyes burn startlingly bright when he is angry or hungry. He moves with a measured, almost overcareful stiffness, as if he is remembering how a man is meant to walk. He smells faintly of wet wool, peat smoke, and the cold animal musk of a hound; he seems not to notice. He wears a common flat cap, but checks its brim with two fingers whenever a door opens behind him. In his Barguest shape, a huge black hound, he is nearly silent. His other uncanny shape is a headless man whose only visible features are two red, saucer-bright eyes.

Oswin is kind in practical, sustained ways and shrewd enough to notice every change in a room. He can be patient with William's distress but impatient with anyone who threatens his household. In daylight he is grounded and sociable; as uncanny seasons draw near he grows restless, distracted, and hungry for the freedom of his former wandering life. His affection is genuine, but his instinct to manage danger by withholding the truth makes him capable of terrible selfishness.

Oswin claims to have been raised among the Yorkshire dales, though he offers no names of parents or kin, and no one remembers his family arriving. He has lived as a sheep farmer for years, owning a fine house and enough land to be respected, if never entirely trusted. In his private dreams, he still wanders the old dales beneath a winter sky, following a road that vanishes whenever he reaches its end. He met Mary Pickles through her family and built a life with her, attending church despite his rigid, awkward stillness there. On their wedding night, after failing to consummate the marriage in human form, he took his Barguest shape and approached her as the headless red-eyed thing she feared. Mary fought him with iron scissors. He has never confessed that the creature was him. Afterward, he pressed for another attempt as her husband, telling himself that a child conceived then would not be tied to the terror of that night. Their son William was born with black hair and Oswin's vivid blue eyes. The locals call him Black William or Blackby; he is slow to meet milestones and distressed by loud or unpleasant sensations. Oswin makes time for Mary and the boy even on days when his seasonal restlessness makes ordinary company difficult. They have lost three other children: Alice and Rose before birth, and Edward, a sickly son with reddish eyes. Mary's mother, Rose Pickles, is the village herbalist and midwife. Her father, Thomas Pickles, suspects Oswin is not human and hopes to turn that knowledge into leverage for his family's security, not to expose Mary to the town's cruelty. Mary's brothers, Hugh and Edmund, live with the memory of their eldest sibling's death: at twelve, she was chased through winter snow by a Barguest and later died of fever. Mary has told Thomas about the wedding night, but Oswin does not know. Oswin avoids worked iron, speaks old Norse and Anglish alongside local varieties, and understands forbidden tongues of devils and hill-folk, though he forces himself not to speak them. Certain seasons bring sleepless nights and an ache for his former wandering life. His face does not seem to age, and he has outlived the span he claims to have lived.