The Smiling Knight was an infamous knight of the Seven Kingdoms. Due to the fact that he smiled during combat, a fact that earned him his nickname, many thought of him as insane. He became notorious as a member of the Kingswood Brotherhood, a group of outlaws who existed during rule of the Mad King Aerys.
He was slain by Ser Arthur Dayne in personal combat, after having briefly fought Jaime Lannister. Jaime compares the Smiling Knight to Ser Gregor Clegane in terms of men's eagerness to defeat him. It is for sparring with the Smiling Knight that earns Jaime his knighthood.