Who is Joffrey Baratheon from A Clash of Kings and what is their importance?

Joffrey Baratheon from A Clash of Kings is a spoiled, cowardly, cruel boy of 13 who comes to rein way before his time. He was born of incest between Cersei and Jaime Lannister, as were his siblings Myrcella and Tommen. Almost everyone believes that Joffrey and his siblings are the children of Robert Baratheon.

He is engaged to Sansa Stark, but treats her horribly. He makes her watch as her father is executed, then makes her visit his head where it is stuck on a pike on the front gates. He often has his Kingsguard beat her when her brother Robb wins a victory. Tyrion is the only one that Joffrey fears. His people hate him. When he rides into town there is almost always a riot.