Lord Tywin Lannister is the head of House Lannister and the richest man in the Seven Kingdoms. His father presided over a period of decline for the house. He frittered away much of their fortune on poor investments and allowed himself to be mocked at court creating a perception of weakness. Their vassals House Reyne rebelled against the house. Tywin put down the rebellion personally, extinguishing their house and re-establishing the fearsome reputation of House Lannister. His ruthlessness gave darker meaning to the common phrase "A Lannister always pays his debts" and was immortalized in the song "The Rains of Castamere".
Tywin served as Hand of the King to King Aerys II Targaryen for twenty years but resigned after the Mad King rejected Tywin's proposal to marry his daughter Cersei to Rhaegar and made his son Jaime a member of the Kingsguard - events which Tywin perceived as insults. During Robert's Rebellion Tywin remained neutral until the end when he & his army marched to the capital, feigning they had come to defend it from the rebellion. After the city gates were opened, Tywin ordered the city to be taken and the royal family eliminated; as a result the city was sacked and the royal family massacred by Ser Gregor Clegane. Meanwhile, Tywin's son killed the Mad King. After the rebellion Robert Baratheon took the throne and married Tywin's daughter Cersei to cement their alliance.