Gregor Clegane

Gregor Clegane is a character in Game of Thrones. He has not been cast as yet, but will appear in the first season.

Ser Gregor Clegane is the elder brother of Sandor Clegane, whom he despises, and is a landed knight. Gregor is huge, the largest man in Westeros, eight foot in height and heavily muscled. He is the most feared man in Westeros with a reputation for ferocity and anger. He is a loyal retainer and servant of House Lannister, and is particularly loyal to Lord Tywin.

In the books
In the Song of Ice and Fire novels Gregor Clegane is a ferocious warrior who wields a sword that most men need to wield two-handed in one hand, and wears armour that is so heavy most men cannot even lift it. He can only ride the largest and strongest horses. It is heavily rumoured that Gregor committed horrific and unspeakable deeds during the Sack of King's Landing that ended Robert's Rebellion.