House Stark is the principal noble house in the North. Their seat is Winterfell. Their sigil is a grey direwolf on a field of white, and their words are "Winter Is Coming." The Starks are descended from Brandon the Builder, a legendary figure from the Age of Heroes thousands of years in the past, who built Winterfell and the Wall. They were Kings in the North until Torrhen Stark submitted to Aegon the Conqueror at the end of the War of Conquest. Since that time the Starks have held the North for the kings of Westeros as Lords of Winterfell and Wardens of the North.
House Targaryen, of the blood of old Valyria, was the royal house of Westeros for some 280 years before being deposed in the War of the Usurper. Their ancestral fortress was Dragonstone before they established the city of King's Landing and their stronghold of the Red Keep within it. Their sigil is a red three-headed dragon, breathing flame on black. Their words are "Fire and Blood".
House Lannister is the ruling house of the Westerlands. Their principle stronghold is Casterly Rock and their symbol is a roaring lion, gold on crimson. Their words are "Hear Me Roar!"
The White Walkers are thousands of years old, coming from the time preceding the Age of Heroes. Born of powerful and untested magic, they were created to protect the Children of the Forest from the First Men, who had waged war on them ever since they had arrived from Essos. However, the White Walkers eventually broke free of the Children's control and became the most feared creatures in Westeros, posing a threat to anything living.
Marc Rissmann is set to appear in Season 8 as Harry Strickland, the leader of the Golden Company. The Golden Company is a sellsword company active in Essos. They are one of the largest and most skilled sellsword companies in the Free Cities, consisting of 10,000 infantry, cavalry, and war elephants. They have the reputation of never breaking a contract, which is quite uncommon among mercenary organizations.
Sarra Frey is one of the many granddaughters of Walder Frey. Sarra was one of the Frey women presented to Robb Stark when he came to the Twins for Lord Edmure Tully's wedding.