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Harald Karstark is a character from Game of Thrones. He is a composite character created specifically for the show, serving as a combination of Arnolf Karstark and Harrion Karstark. He is Rickard Karstark's last surviving son and the Lord of Karhold.
In the novels Harrion is Lord Rickard's last surviving son. Since Harrion is the oldest child, Harald might be the TV series version of Eddard Karstark, due to his age. He's described as ruthless and calculative.
Harald is the son of Rickard Karstark and younger brother of Harrion and Torrhen Karstark. 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, leading the Karstarks back to Karhold. He has since allied himself with Roose and Ramsay Bolton.
Harald first appears in the episode "Oathbreaker" before both Roose and Ramsay 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 after King Robb's execution of his father.
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 even chastises Wolkan for failing to address Ramsay as "Lord." He also makes no effort to dissuade Ramsay from murdering Walda and her baby, even though their deaths will most likely spark a conflict with House Frey, the current Great House of the Riverlands.
He fought the Battle of the Bastards, and he lost alongside the whole Bolton forces. It is unknown whether he is killed in action, executed or imprisoned for his crimes, or defected to Starks (though less likely).