Cersei Lannister is a character in HBO's series Game of Thrones.
Cersei is the only daughter and eldest child of Tywin Lannister who arranged to have her wed Robert Baratheon after the overthrow of the Targaryen dynasty, thus ensuring the family's position at the centre of Westeros' political map. Unbeknown to her father, Cersei was having an incestuous affair with her twin brother Jaime, whom she would rather have wed, but she decided to make the best of her marriage to Robert, for the sake of the family, though the secret relationship with her brother continued. She completely soured on her new husband when, on their very wedding night, he drunkenly climbed into bed with her and uttered the name of the woman he was originally supposed to marry, Lyanna Stark, before she was murdered by the Targaryen king, and for whom he went to war to avenge. She has despised him from that day onward.
Cersei's relationship with her brother has resulted in all three of her children being born of incest, unbeknownst to anyone but they. Her disdain for her husband has her regularly spending time with Jaime, and during a visit to Winterfell, one such meeting was observed by Bran Stark. In order to keep their relationship secret, Jaime pushed the boy from the window of the high tower in which they were hiding, hoping for the fall to kill him.
Their secret has come under threat numerous times. Jon Arryn, the late Hand of the King, came dangerously close to discovering the truth before the twins conspired to have him killed. Eddard Stark, the man named by the King to replace Arryn, investigated the death of his old friend and eventually came across the entire truth. Foolishly, he confronted Cersei with it, offering her the chance to flee with her children before he told the King. Instead, she arranged for the King to become mortally wounded in a hunting accident, using his squire and her cousin, Lancel Lannister, to unwittingly get the King drunk enough to dull his reflexes.
With the King dead, she quickly arranged for her eldest son Joffrey Baratheon to ascend to the throne before Ned Stark can make his move against them.
Cersei is cunning and manipulative in the pursuit of her goals, and those goals are all geared toward her desire to secure power and prosperity for herself and her family. She is cold-hearted and uncaring for anyone that isn't a Lannister, and has no problem with killing innocent children if it means her own will be better off. Making an enemy of her may be one of the last things you ever do.