Arnulf Karstark

Arnulf Karstark is a recurring character that appeared in the end of Season 5 and the beginning of Season 6. He is the younger brother of Lord Cregan Karstark.

Season 5
Arnulf is first mentioned when Stannis begins his march toward Winterfell in "The Dream". Stannis explains to his advisers that Arnulf leads a loyalist section of the Karstark forces, and they current marching to meet with the Baratheon host.

Arnulf first appears in episode 10 of Season 5, when his army finally links up with Stannis's in the blizzard. In Winterfell, Roose Bolton sends Hosteen Frey to lead a host to defeat Stannis, with Stannis's position marked on a map given by an informant in the Baratheon camp.

Season 6
Arnulf is captured by Stannis's troops, and the king explains that he knew all along Arnulf was the traitor and he had purposely allowed Arnulf to send the map. Stannis then explains he has plans for Arnulf.

In his final appearance in "King's Blood", Theon Greyjoy is supposedly sacrificed in the burning of the weirwood, but the sacrifice was actually a glamoured Arnulf, made to look like Theon.