House Karstark of Karhold is a cadet branch of House Stark, founded a thousand years ago when Karlon Stark was granted lands east of the Lonely Hills for putting down a rebel lord. The keep he raised there was called Karl's Hold, and over generations the name and the line both shortened, until the lords of the place styled themselves Karstark and their fortress Karhold.
The kinship is far enough back that no one now alive remembers a Stark and a Karstark sharing a hall as cousins, but the Karstarks have never forgotten it. They still keep the Old Gods, still wear the long beards their forefathers wore, and still answer Winterfell's call with a swiftness their lands' distance does not promise. Lord Rickard Karstark sits at Karhold with his sons Harrion, Eddard, and Torrhen, and his daughter Alys.