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 dowager 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, although it is heavily implied that they are illegitimate and a result of her incestuous relationship with her brother Jaime. She is noted for her great beauty.

Winter is Coming
In King's Landing, the capital city of the Seven Kingdoms, the bells ring out to commemorate the death of Lord Jon Arryn, the Hand of the King. As the silent sisters prepare his body for burial, Cersei looks on. She is worried that Jon discovered something he shouldn't have, and may have told someone. She confesses her fears to her twin brother, Jaime Lannister, but he tells her not to worry, since if the King knew the truth they'd be dead already.

The royal party arrives and Lord Eddard presents his household to King Robert. After initial greetings, Robert demands to see the crypts and pay his respects, to Cersei's impatience. After Robert and Eddard depart, Cersei sends Jaime to find their younger brother Tyrion.

A great feast is held and Cersei and Catelyn exchange cordial words. Cersei seems more impressed by Sansa, thinking that she will be a great hit at court.

The next morning Eddard and Robert go hunting, leaving the castle largely empty. Bran indulges in some climbing, but whilst climbing a broken tower he hears noises. He finds Cersei and her brother engaged in sexual intercourse, but is caught watching by Cersei, and Jaime grabs him. He asks how old Bran is. When Bran replies "ten", Jaime sighs and pushes him out of the window, musing "the things I do for love."

The Kingsroad
Tyrion breakfasts with his other family members he tells them that Bran is expected to live and notes Cersei and Jaime's guarded reactions.

Cersei visits Catelyn Stark, sitting by her son's side, and tells her that her first child, a beautiful, black-haired boy, was stillborn. She and Robert were grief-stricken and her prayers and tears were for naught. She offers to pray for Bran's survival, hoping that this time the gods will listen.

Arya is taken before the King, who is angered that a minor fracas has become a major incident with his wife and new Hand placing blame on one another's children. He decides to let Ned discipline Arya whilst he will do the same to Joffrey, but to appease Cersei he agrees that the direwolf must be executed.

Lord Snow
Cersei talks to Joffrey about the incident on the river. She tells him that when he is king, the truth will be whatever he decides it will be. Joffrey tells her that he thinks there will be trouble with the Starks, and that when he is king he'll double their taxes, force them to contribute to a standing army and, if they should object, he'd crush them. Cersei points out that the North is too vast and wild to be militarily conquered by outsiders, and as king he needs to be cleverer. She also advises him to be kind to Sansa to avoid storing up trouble for later, and concludes that "anyone who isn't us is an enemy."

A raven arrives in King's Landing with news of Bran's recovery. Cersei fears he will expose their secret, but Jaime comforts her.

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.