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Harwin is a member of the Household guard at Winterfell. He is the son of Hullen, the Master of Horse.
Harwin and his father are among the household guard that accompany Eddard Stark south to King's Landing when Robert Baratheon names him Hand of the King. When Gregor Clegane "the Mountain that Rides" began raiding the Riverlands, Harwin is among the 100 men under Beric Dondarrion that Eddard Stark sends to arrest Gregor. However they are ambushed at the Mummer's Ford and Beric is wounded by the Mountain's lance. Beric's squire Lord Edric Dayne pulls from the water and stands over him while the fighting happens, though Beric dies. The Red Priest Thoros of Myr, who is a friend of Beric, performs the last kiss on Beric, a rite of R'hllorism. To his astonishment Beric is brought back to life. Beric rallies the survivors to fight the Lannisters' invasion of the Riverlands. However when King Robert dies the Lannisters usurp the throne, making Beric and his men outlaws. Eddard Stark is executed by the Lannisters' King Joffrey Baratheon. Eddard's son Robb Stark arrives in the Riverlands to fight the Lannisters, and on hearing of his father's death the Northmen and Rivermen at Riverrun proclaim him King in the North.
The war between the Lannisters and Starks leaves the Riverlands in ruins. Beric raids the Lannisters' foraging parties. Though some of his men die, others join Beric's force, who become the Brotherhood without Banners. Though Beric and his men were originally sent to fight the Lannisters, the Brotherhood fights all who oppress the smallfolk. Beric is killed five times but each time Thoros revives him.
The Brotherhood encounter Arya Stark, younger daughter of Eddard Stark, and Gendry, a bastard of Robert Baratheon. Harwin recognises her. As they need money to buy food for the smallfolk, the Brotherhood decide to ransom her to Robb Stark, though Harwin would prefer her to be simply returned to her brother. When Arya tries to escape the Brotherhood on horseback Harwin is the only one who can keep up with her, and tells Arya her skills in horse-riding remind him of her aunt, Lyanna Stark.
When Beric is killed in a battle with the Mountain's brother Sandor Clegane "the Hound" Thoros revives him for the sixth time. Harwin denies Beric has been resurrected, claiming Thoros is merely a competent healer. Arya later runs from the Brotherhood and is captured by Sandor.
In what becomes known as the Red Wedding, Robb Stark, his mother Catelyn Stark, and most of their army are massacred and Catelyn's brother Edmure Tully captured by their treacherous vassals, the Freys and Boltons, who have secretly joined the Lannisters. Catelyn is stripped and thrown in the river after a day and a night. Arya, while warging into her direwolf Nymeria, finds her mother and drags her out of the water. She is found by the Brotherhood and Harwin begs Thoros to bring her back. However Thoros claims she has been dead too long. Beric then performs the last kiss on her, and Catelyn rises, while Beric dies a seventh and final time. Leadership of the Brotherhood is taken by Catelyn, who becomes known as Lady Stoneheart. Stoneheart's mission, to take vengeance on all those she believes connected to the Lannisters, leads the outlaw band to fracture. However Harwin is listed as still being a member of the Brotherhood in the Appendixes. When Brienne of Tarth is captured by the Brotherhood, a young northman wearing a greasy sheepskin jerkin interprets Stoneheart's rattled speech for her, as her cut throat makes it difficult to interpret.