Jaime Lannister is Cersei's twin brother and secretly the father of her three children, Joffrey, Myrcella and Tommen. Known as "The Kingslayer," it was Jamie who betrayed his oath as a Kingsguard when he slew "the Mad King" Aerys Targaryen, by order of his father, Tywin Lannister. Jaime served King Robert with seething hatred as he watched the drunken man constantly shame his sister; after Tyrion was captured by the Starks, Jaime once gain heeded his father's wish and rode to war for his family...only to be captured by young Robb Stark.
With the Lannisters holding Sansa Stark hostage in King's Landing, Jaime is still of value alive. But even without his sword, The Kingslayer is considered one of the deadliest men in the Seven Kingdoms...with good reason.