Tywin's eldest child by mere moments, Cersei is the twin sister of Jaime Lannister and matches his beauty, with golden hair, bright green eyes and a lush, curvaceous body. After Robert's Rebellion she married the new King, Robert Baratheon, and became Queen of the Seven Kingdoms. A willful and ambitious woman, Cersei resents the restrictions put on her for her sex. She is always concerned for the safety of her royal children, Joffrey, Myrcella, and Tommen, and stops at nothing to protect them.
Cersei has shared an incestuous relationship with her brother Jaime since they were youths, and continued it throughout her fifteen-year marriage to King Robert, made possible both by Cersei's craftiness and Robert's philandering nature. Her three children were all a product of this incest, rather than her husband King Robert; this secret, when discovered by Jon Arryn, Eddard Stark and Stannis Baratheon, was used as a political tool during the War of the Five Kings.

Throughout the series, Cersei has increasingly climbed the ladder of power. After arranging the death of her husband, Cersei put her 13-year-old son Joffrey on the throne, despite accusations of bastardy, and ruled as his Queen Regent. However, Tywin promptly put Tyrion in the position of Hand of the King, and from there Tyrion was able to curtail much of her policy, a practice which Tywin continued upon his assumption of the office. They did so because Cersei is not, in fact, a very good ruler. Her brother Jaime describes her thusly:

[Cersei] liked to think of herself as Lord Tywin with teats, but she was wrong. Their father had been as relentless and implacable as a glacier, where Cersei was all wildfire, especially when thwarted. ... She does not lack for wits, but she has no judgment, and no patience.

When Joffrey and Tywin were murdered in quick succession, the checks on her claim to power were cleared. She resumed her position as Regent over her son, the eight-year-old King Tommen, who was entirely obedient to her will, and became queen in all but name. As the eldest child of Tywin, she was also acknowledged as the Lady of Casterly Rock by her uncle Kevan. Thus empowered and unrestricted, Cersei felt she had the opportunity to prove herself an even better ruler than her father. She began her new regime by filling all of the positions on Tommen's Small Council with her own supporters and agents, disregarding the previous arrangements made by Tywin and well-meaning (if blunt) advice from Kevan. The changes she made to the small council along with some of the men she choose to fill its positions were not held in high regard by many of the lords and smallfolk which led to many referring to the small council as "the smallest council".

Cersei grew to suspect that her powerful Tyrell allies, now relatives through Tommen's marriage, were trying to seize control of the kingdom. She commenced a campaign of intrigue to remove all the Tyrells in King's Landing from positions of influence and authority, including her son's wife Margaery, and Loras, a member of the Kingsguard. She thus alienated Jaime, who abandoned her to her political paranoia. She refused to honor the debts owed by the Crown, angering powerful institutions such as the Iron Bank of Braavos and the Faith. With the monies owed, she constructed a new royal fleet of warships and gave command to Aurane Waters, a sellsail of considerable charm but questionable expertise and loyalty. In an attempt to alleviate the crown's debts and gain more protection from her purported enemies, she allowed the High Septon to revive the Faith Militant, ignoring its history of causing trouble for monarchs. She also took the disgraced former Maester Qyburn into her service, using him as a torturer and allowing him to conduct immoral experiments on human subjects.

She tried to ensnare Margaery Tyrell by telling Osney Kettleblack to seduce her, but the attempt failed as Margaery did not respond to his seduction with anything other than teasing. Thereafter she tortured Margaery's singer (the Blue Bard) into admitting to having sex with Margaery Tyrell and her companions; she also had Osney (who was only truly interested in sex with Cersei) go to the Great Sept and confess to having bedded Margaery Tyrell. This led to Margaery's arrest by the High Septon of the Faith. However, Cersei's plan backfired when Osney, arrested by the High Septon and tortured, revealed the truth about Cersei's plan, as well as the fact that the previous High Septon had been murdered on her command. Cersei was arrested and detained to await trial. Her own ministers abandoned her and seized control of the government while she was kept prisoner in the Great Sept of Baelor. They recalled her uncle Kevan to fill her position as regent, while her new lord admiral absconded with the costly new fleet. Accused of capital crimes, her only hope lay in a Kingsguard champion to stand for her in a trial by combat. She sent an emotional summons to Jaime, which he burned and to which he has not replied.

Cersei has been haunted since childhood by a prophecy given to her by a maegi. This prophesy correctly predicted the death of a childhood friend, her unrequited love for Rhaegar Targaryen, and her marriage (and mutual infidelity) with Robert. It also predicts that all three of her children will be crowned, but that all three will die before her. Everything that she has will be taken away by a younger and more beautiful queen, and then her valonqar (Valyrian for "little brother" or "younger brother") will strangle her to death. Cersei believes (whether correctly or not) that the younger, more beautiful queen is Margaery Tyrell and the valonqar is Tyrion Lannister.

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