Joffrey Baratheon was the Crown Prince of the Seven Kingdoms, and became king when his "father" King Robert died. He is a short tempered, cruel boy who is married to Sansa. He kills Ned Stark stating that he performed acts of treason, and is unaware that Jaime is actually his father, therefore, he is not the rightful king.

Game of Thrones Quotes

Tyrion: Let me give you some advice bastard. Never forget what you are. The rest of the world will not. Wear it like armor, and it can never be used to hurt you.
Jon: What the hell do you know about being a bastard?
Tyrion: All dwarfs are bastards in their father's eyes.

I will hurt you for this. A day will come when you think you are safe and happy, and your joy will turn to ashes in your mouth. And you will know the debt is paid.

Tyrion Lannister