Ser Gregor
Clegane is the head of House Clegane, a knightly house from the Westerlands,
and the elder brother of Sandor Clegane. When Sandor and Gregor were
children, Gregor held his brother's face in a fire for playing with one of his
toys without permission, horrifically scarring him. Gregor is a landed
knight rather than a full-fledged lord. Gregor is monstrously huge, said to be
the largest man in Westeros. He is feared throughout Westeros with a
reputation for ferocity and anger. He is a loyal retainer and servant of House
Lannister, particularly to Lord Tywin.
Numerous
servants have gone missing at the Clegane keep over the years, apparently
killed by Gregor for minor slights in one of his infamous fits of rage, and
their deaths then covered up. Gregor's own father and sister died under dubious
circumstances as well, and Sandor thinks that Gregor may have killed them as
well. Their sister died when Sandor was too young to remember her well, and
their father died years later in what was said to be a "hunting
accident". At the request of Tywin Lannister, Prince Rhaegar
Targaryen himself anointed Gregor as a knight, a great honor, even though
Gregor was an infamously dishonorable man.
Ser Gregor Clegane has remained faithful to the House Lannister by defeating every great enemy of the House. For instance, he killed a sparrow boy brutally, who supported a very new religion, under the command of Queen Cersie Lannister.
In conclusion, Ser Gregor Clegane is an man of dishonor, filth, and hatred for love, making him a merciless man. (Source:-http://gameofthrones.wikia.com/wiki/Gregor_Clegane)