Cersei Lannister is the eldest daughter of Lord Tywin Lannister and Lady Joanna. She is the widow of King Robert Baratheon and the Queen Regent of the Seven Kingdoms. Her son Tommen currently sits the Iron Throne of the kingdom and continent, while her only daughter, Myrcella Baratheon, is currently at Dorne.
Cersei, a cruel, ambitious woman, is the daughter of Tywin Lannister, former leader of House Lannister. She has a twin brother named Jaime, with whom she carried on an incestuous relationship for many years. This bond resulted in Cersei's three children, Joffrey, Myrcella Baratheon and Tommen I Baratheon, although most of the populace, including her husband the king, believe that the children are the rightful heirs to the throne as Robert's issue. It is this secret that causes much of the murder and unrest in the novels. She married Robert Baratheon after he became King of the Seven Kingdoms, but grew to despise him due to his unresolved feelings for Lyanna Stark. After Robert's death she became regent, first to Joffrey and then to Tommen. Cersei is described as a stunningly beautiful woman.
Due to a childhood prophecy, she knew early in life that she was fated to rule the Seven Kingdoms, and embraced this destiny. The prophecy also stated that she would see all of her children die before her own death, and would lose everything to a younger queen. However, upon coming into her power, the clever Cersei began taking steps to avert the latter part of the prophecy, which sets the stage for many of her actions throughout the tale.
During her earlier life at court she was content to let Robert hunt, drink and whore while she continued to have an affair with her twin brother Jaime Lannister. She often found herself in agreement at court with Robert's long-time friend Alek Nocher who thought that Robert was far too soft and forgiving to the ironmen after the Greyjoy Rebellion. During the War of Five Kings she acted as Queen Regent and after Tyrion arrived to act as Hand she was usually at odds with him. After Joffrey's sudden death she became paranoid that Tyrion would return to kill her second son Tommen Baratheon and stole power after Tywin Lannister was murdered. She formed her own small council which proved for the most part unable to deal with the Kingdom's many issues such as the sudden aggression of the ironmen lead by Euron 'Crow's Eye' Greyjoy.
She tried to destroy the influence that Queen Margaery had on her son Tommen and even placed false evidence to have her arrested for crimes of adultery which caused Mace Tyrell, Lord of Highgarden, Margaery's father, to abandon his siege of Storm's End. She was later arrested herself by the Faith and accused of adultery and made to shave her head bald and walk naked from Baelor's Sept to the Red Keep whilst being mocked and japed at by an unsympathetic crowd. She is currently back at the Red Keep and was last seen enjoying a pleasant dinner with her son Tommen and her uncle Kevan Lannister.
Cersei is awoken to the news of her father's death, and is filled with rage upon learning that the deed was done by none other than Tyrion. She meets Jaime in Tywin's chambers, demanding that the vast network of tunnels under the Red Keep be searched for any signs of her imp brother. Cersei orders the Kettleback brothers to dispose of Shae's corpse, and keep her location in Tywin's bed as a secret. Upon sending informants to search Tyrion's cell, only a Tyrell coin is found, prompting suspicion and a hunch that the Tyrell's may have been behind Tywin's murder. Cersei considers her uncle Kevan Lannister for the role of the new Hand of the King, distrustful of Lords Mathis Rowan and Paxter Redwyne, and angered by Jaime's refusal to take up the position. During Tywin's funeral, in which she accompanies Tommen, Cersei is frustrated by Lady Olenna Tyrell's underhand remarks. She is, however, attracted to Lady Taena Merryweather's offers to help bring Tyrion to justice. Cersei later meets with Kevan, proposing that he become the new hand, and is infuriated when he refuses, throwing wine in his face.
Cersei begins to hire out mercaneries with the task of finding and killing her convicted brother, but is only further disappointed when they return with the severed heads of random dwarves, none bearing any resemblance to Tyrion. During the wedding of her son, Tommen, to Margaery Tyrell, Cersei has the ceremony under close supervision in order to prevent any danger from reaching her son. She remains passive and coldly isolated for much of the wedding, finding solace only in Lady Taena, who gives her information regarding one of Margaery's maids spying on the Queen for her. That night, Cersei sets the Tower of the Hand on fire and watches it burn with satifaction.
Cersei eventually settles on Harys Swift as her new hand and assembles an entirely new Small Council consisting of new additions like Aurane Waters and Orton Merryweather. Her hatred for Margaery also grows due to her influence over Tommen, and plots to overthrow her. She seduces Osney Kettleback and commands him to try to initiate sexual interaction with the young queen, in order to expose her as a harlot and ruin her image with the public. She sends her brother, Loras Tyrell, to try to take Dragonstone from Stannis, to Margaery's horror. After she and Qyburn torture the Blue Bard, a known companion of Margaery, for information (though lacking validity) concerning Margaery's various "partners", they are able to compile a list of names including Osney, Jalabhar Xho, Bayard Norcross, Ser Tallad the Tall, Hugh Clifton, Hamish the Harper, Lambert Turnberry, and Mark Mullendore.Cersei then frames Margaery, who is imprisoned by the Faith shortly afterwards.
When Cersei visits Margaery in her cell, they engage in a heated confrontation, before Cersei herself is arrested and charged with treason by the High Septon after Osney is interrogated, revealing the Queen's plans. She is forbidden from having visitors, and is furious with rage.