Lord Randar Bolton is a major character in the fifth and sixth seasons, and was previously a recurring character in the third and fourth seasons. He is the older brother of Roose Bolton and the head of House Bolton of Strathmore, a cadet branch of House Bolton that he founded after he was given the castle of Strathmore following his role in the Battle of the Trident during the War of the Usurper.
Alongside his brother, Randar fought for Robb Stark during the War of the Five Kings. Even after Roose betrayed Robb and became the new Warden of the North, Randar initially continued to ally with him, with the promise that he'd give him the Dreadfort once he rebuilt Winterfell. However, Randar's disgust at Ramsay Snow's torturing of Theon Greyjoy into insanity lead him to abandon Roose and eventually become a member of Grayburn's Legion. He is currently trying to broker an alliance with House Bracken of Stone Hedge (one of House Tully's strongest bannermen who submitted to the rule of the Iron Throne after the Red Wedding) by marrying one of Lord Jonos Bracken's daughters, in order to further bolster Grayburn's Legion's forces and gain the support of the Riverlords.
Following the death of both his brother and later the death of Ramsay at the Second Battle of Winterfell, Randar is the last surviving trueborn male that is directly of the Bolton line, and is therefore now the new Lord of the Dreadfort.