Cersei Lannister

Cersei Lannister, also (rarely) called Cersei Baratheon, is a major character in Game of Thrones. She is played by Lena Headey.

Cersei is the Queen of the Seven Kingdoms. She is the only daughter of Lord Tywin Lannister, the twin sister of Ser Jaime Lannister and the elder sister of Tyrion Lannister. She is the wife of King Robert Baratheon, a political marriage agreed to by Robert in thanks for her father's late switching of sides in the civil war which brought Robert to power. With Robert she has three children: Joffrey, Myrcella and Tommen. She is noted for her great beauty.

In the books
In the Song of Ice and Fire novels, Cersei is a beautiful but also somewhat cold woman who does not get on with her husband, whom she considers to be a fat oaf. However, she dotes on and loves her three children. She is politically skilled, but frustrated that her options for power are limited by her gender. Cersei is close to her brother Jaime, but is cowed by her father and despises Tyrion, whom she blames for killing her mother Joanna, who died in childbirth.