Harald Karstark

After the deaths of his father Lord Rickard Karstark and his brothers, Harald inherits Karhold and command of his house, which has sided with the Boltons in the war for the North.

He shares Ramsay Bolton's plan to storm Castle Black and capture Sansa Stark, and advises Lord Roose Bolton to kill Jon Snow, whom he sees as the last remaining threat from Eddard Stark's line. He makes no secret that his hatred of the Starks stems from his father's execution.

Harald is present when Ramsay murders his own father, Lord Roose, and raises no objection when Ramsay lies about his death afterward or orders the killing of Lady Walda and their newborn son. When Maester Wolkan is slow to call Ramsay "Lord", Harald rebukes him for it.

Together with Ramsay, he meets the Smalljon Umber to secure his support. Harald doubts the wildlings could ever take Winterfell, though Umber believes they could under Jon Snow's command.

He rides with Ramsay to parley with Jon Snow and Sansa Stark before the battle, and returns with him to the castle when the talks collapse. In the fighting he leads a cavalry charge straight at Jon Snow.

The Stark cavalry intercepts Harald's riders, and Ramsay's own arrows fall on them. Harald dies in the slaughter, before the walls of Winterfell.

There is no Harald Karstark in A Song of Ice and Fire. After Lord Rickard Karstark and two of his sons die in the war, it is another son, Harrion, who inherits Karhold.

In that sense the show's character is an invention of the adaptation: in the books the succession at Karhold follows a different path, without the House Bolton storyline that drives Harald on screen.