Harwin has been a member of the household guard at Winterfell since his youth, and he possibly worked as a stable boy before that, as his father, Hullen, was the Master of Horse at Winterfell. Harwin was at Winterfell when Lyanna Stark was still alive, as he remembers her skill at horse riding. He was also one of the guards privy to the rumours of an affair between Lord Eddard Stark and Ashara Dayne.
Arya remembers that it was Harwin who taught her how to ride when she was younger, often leading her pony round the Winterfell yard. He also taught Robb Stark and Jon Snow how to joust, often riding at quintain with them. He was known around the castle to drink too much during feasts.
A Game of ThronesEdit
Harwin rides out with Eddard Stark to execute the deserter Night's Watch ranger Gared. On the way back they encounter a dead direwolf, killed by a stag, and it's 6 pups. He says that it would be impossible to keep them, after Robb asks his father whether or not they can take them back.
Harwin travels south to King's Landing with Eddard after he is offered the post of King's Hand of Robert Baratheon. Arya resents him for not standing up to Sandor Clegane when the Hound murders the butcher's boy, Mycah. Harwin takes part in the Hand's Tourney, but he is unhorsed in his first tilt by the Kingsguard knight, Ser Meryn Trant.
Harwin is dispatched by Eddard under the command of Lord Beric and Thoros of Myr. He fights and survives in the Battle at the Mummer's Ford, and becomes a member of the Brotherhood without Banners. It is unknown if he abandons the Old Gods for the faith of R'hllor as most of the Brotherhood do.
A Storm of SwordsEdit
Harwin is present at the inn when Tom of Sevenstreams, Lem Lemoncloak and Anguy bring in the disguised Arya Stark, and Harwin identifies her after she breaks the nose of Lem. He is a hardened man, much thinner than he had been before. He tells Arya that her riding skills remind him of those possessed by her aunt, Lyanna. This isn't the first time that Arya was compared to Lyanna. He is the only member of the brotherhood capable of catching Arya after she tries to escape on horseback.
After the Red Wedding Harwin begs Thoros to revive the corpse of Catelyn Stark, but he refuses. Instead Beric gives his life to bring back hers, and Harwin becomes a member of the Brotherhood under the command of Lady Stoneheart.