Harald is the youngest son of Lord Rickard Karstark and the new head of House Karstark and Lord of Karhold after his father's execution by Robb Stark.
Harald is the son of Rickard Karstark and younger brother of Harrion and Torrhen Karstark. After his father and brothers follow Robb Stark into war against House Lannister, Harald remains behind at Karhold as its acting Lord. However, following the deaths of his two brothers earlier in the war, and then his father's execution at the hands of Robb Stark for treason, Harald becomes the new head of House Karstark and forsakes his allegiance to Robb, summoning the Karstarks back to Karhold.
Harald appears before Lord Roose Bolton and Roose's son Ramsay Bolton at Winterfell, reporting that his men had found the remains of the Bolton hunting party that was sent to track down Sansa Stark and Theon Greyjoy. Harald counsels Roose that, despite being a bastard, Jon Snow, as Ned Stark's last surviving son, presents a threat to the Boltons' rule of The North. He affirms his house's loyalty to House Bolton, saying that he will never follow a Stark again after Robb executed his father, Lord Rickard Karstark. When Maester Wolkan arrives to announce that Walda Bolton has given birth to a boy, Harald congratulates Roose. However, despite the fact that Roose avenged his father by killing Robb, Harald shows no surprise and makes no attempt to intervene when Ramsay kills Roose shortly afterward. He then chastises Wolkan for failing to address Ramsay as "Lord", and makes no effort to dissuade Ramsay from murdering Walda and her baby.
Harald and Ramsay later meet with Smalljon Umber, the new Lord of Last Hearth. Harald notes that the Umbers are loyal to House Stark, but Smalljon retorts by noting that the Karstarks share blood with the Starks, yet Harald is now pledged to Ramsay Bolton. Harald casts doubt on the possibility of the wildlings taking Winterfell, but Smalljon says that, with Jon Snow in command, it would be possible. When Smalljon brings in Osha and Rickon Stark, he quips that Harald has an interest in young boys, visibly angering him.
When news of the Karstarks and Umbers declaring for Ramsay reaches Sansa at Castle Black, she says the Umbers can hang for giving her brother to Ramsay but the Karstarks did so without knowing that they had "another choice." However, Davos Seaworth reminds her of Robb's folly when he executed Harald's father. Harald accompanies Ramsay to a parley with Jon Snow and Lady Sansa Stark, and departs with his Lord back to Winterfell when negotiations turn sour. He later leads his army into battle at the Battle of the Bastards, and leads the cavalry charge when Jon is standing all by himself in the middle of the battlefield. However, they are met head on by the Stark cavalry, and fired upon by their own archers on Ramsay's orders. Harald is killed in the ensuing carnage.
In a meeting with the Lords of the North, following Harald's death, his daughter, Alys Karstark has become the new Lady of Karhold. Despite Harald's treachery, Jon forgives House Karstark when Alys swears fealty to House Stark.
Ever since his liege lord, Robb Stark, executed Harald's father for insubordination, Harald Karstark has supported the Bolton claim on the North. Like Ramsay, he has a preference for extreme measures.
Harald is shown to be a calculating, calm and vengeful man who harbors a deep hatred for House Stark after Robb executed his father, even though his house is a cadet branch of House Stark; the founder of his house had himself been a younger Stark son who earned his lands and titles as a reward for helping to put down a rebellion by House Bolton. Despite the fact that Roose had killed the man who executed his father, Harald inexplicably shows no surprise and makes no attempt to intervene when Roose is killed by his own son Ramsay. He also makes no effort to dissuade Ramsay from murdering Walda and her baby as well, even though their deaths would have most likely led to a conflict with House Frey had they ever found out before Ramsay's defeat.