Harrion Karstark

Eddard Karstark is an unseen character mentioned in the second season. Eddard Karstark is a son of Rickard Karstark, Lord of Karhold and head of House Karstark. He is fighting 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 Eddard was killed fighting by Robb's side.

In the books
In the Song of Ice and Fire novels, Eddard Karstark is the third son of Lord Rickard, and also joins his father and brothers, Harrion, the eldest, and Torrhen, the second son, as part of the Northern forces in the War of the Five Kings. Eddard was named after Eddard Stark, Lord Rickard's liege lord at the time of his birth.

However, he's killed by Jaime during the Battle of the Whispering Wood alongside his brother Torrhen as the Kingslayer attempted to cut his way through Robb Stark's personal guard (which included both brothers, among others of the Northern houses) in order to slay the young king himself.