HOUSE LANNISTER
Tywin Lannister (Charles Dance): Patriarch and lord of Casterly Rock.
Cersei Lannister (Lena Headey): Wife to King Robert Baratheon and daughter of Tywin
Jaime Lannister (Nikolaj Coster-Waldou): Brother to Cersei, head of the King's Guard, also known as the Kingslayer, though seldom to his face.
Tyrion Lannister (Peter Dinkledge): Known as “The Imp”, Tyrion is a dwarf, Tywin's bastard son and therefore brother to Jaime, but his small stature belies a quick, incisive mind and an ability to turn most situations to his advantage.
HOUSE BARATHEON
Robert, King of Westeros (Mark Addy): Husband of Cersei Lannister and friend to Ned Stark, Robert sits on the Iron Throne at the opening of the series. He also, as noted above, requests Ned to be his Hand after the death of the previous holder of that office.
Joffrey Baratheon (Jack Gleeson): Son (ostensibly) of Cersei and Robert
HOUSE TARGARYEN
Viserys (Harry Lloyd), eldest son of the late king Aerys II, who was killed by Jaime Lannister and was known as “The Mad King”. Viserys plans to come back from exile to Westeros and take the Iron Throne, his father's seat of power, which he sees as his right.
Daenerys (Emilia Clarke): sister to Viserys, her original role is to marry the Dothraki Horse Lord, Khal Drogo, in return for the chieftain helping Viserys retake his throne by force.