Harrion Karstark

"I lost one son fighting by your son's side."

- Rickard Karstark to Catelyn Stark

Harrion Karstark is an unseen character mentioned in the second season. Harrion Karstark is a son of Rickard Karstark, Lord of Karhold and head of House Karstark. He fought alongside his father for King Robb Stark in the War of the Five Kings. He is killed during the war.

Season 2
Rickard tells Catelyn Stark that Harrion was killed fighting by Robb's side.

Season 3
Lord Rickard attempts to justify his killing of Willem and Martyn Lannister as vengeance for the deaths of Harrion and Torrhen although Robb Stark does not accept his rational and executes him for murder.

In the books
In the A Song of Ice and Fire novels, Harrion Karstark is the eldest son of Lord Rickard, and also joins his father and brothers in the War of the Five Kings.

Harrion however, takes part in the Battle of the Green Fork and taken as a hostage, while it is his brother Eddard who is killed by Jaime Lannister during the Battle of the Whispering Wood. Eddard serves alongside his brother Torrhen as part of Robb Stark's personal bodyguard. When Jaime realizes that his army is lost during the Battle of the Whispering Wood, he makes a final push to single-handedly carve his way through the Northern army in an attempt to kill Robb Stark in single combat. Jaime manages to reach Robb's personal bodyguard and kill several of them, including Eddard and Torrhen, before being knocked unconscious.

Harrion is kept hostage at Harrenhal until the northern prisoners are released by Arya Stark and Jaqen H'ghar and Brave Companions turn on Ser Amory Lorch and deliver the castle of Lord Roose Bolton. Later Roose sends him, Robett Glover and Helman Tallhart to take Duskendale. They are defeated by the Lannisters. Harrion is taken captive, and his current whereabouts are unknown.

Following his father's execution by Robb Stark, Harrion becomes Lord of Karhold. It is unknown whether Harrion is aware of that, and if he is still alive.

Eddard Karstark was apparently switched with his brother Harrion Karstark (or at least, the names were switched) so TV viewers who haven't read the books would not be confused that more than one character named "Eddard" exists, even though George R.R. Martin made it a point in the books to re-use certain names for minor characters, i.e. to emphasize that "Robert Baratheon" isn't the only person named "Robert" in the entire continent of Westeros.

Ramsay Snow briefly masquerades as Harrion during one of the mind games that he plays on Theon. This isn't mentioned in the books, as virtually all of Theon's torture takes place off-page, and is only alluded to.