Sandor Clegane is a member of House Clegane, sworn to House Lannister. He is the younger brother of Ser Gregor Clegane, whom men call the Mountain that Rides. He is nicknamed The Hound for his savage nature and unquestioning obedience to his masters and for the three dogs featured in his family's arms. As a child, Sandor received gruesome facial burns when Gregor shoved his face into a brazier. He came to loathe fire, his brother, and the hypocrisy of knighthood in general.
Sandor Clegane, like most of his family, spent much of his life in service to the Lannisters, though his younger years were filled with strife due to the brutality of his elder brother, Gregor. When their father perished in a suspicious hunting accident, Sandor left the family lands to join the Lannister household. He served as a body guard for Joffery Baratheon, and was later appointed to the Kingsguard, despite never taking his knightly vows.
War of the Five Kings
As the Baratheon-Tyrell host approaches, largely unchallenged, riots break out in the city. While they are easily contained early on, they become more and more prevalent as the days go by. One particularly brutal one sees a band of men with torches surround the Hound, trying to slay him even as the hovels around them burst into flame. Sandor slays the rioters, and escapes the burning wreckage, but decides that night to flee the city. He stops briefly in the chambers of Sansa Stark, before slipping out through a postern gate and heading north.
Journey in the Riverlands
While in the Riverlands, the Hound comes across the missing Stark sister, Arya, as she flees from the brotherhood without banners. He kidnaps her, taking her to Robb Stark for ransom. They meet at the Twins, and though Robb is skeptical, knowing the Hound as a loyal servant of the Lannisters, he accepts his sister gratefully and pays the ransom. Coin in hand, Sandor leaves, disappearing into the Riverlands with his horse, sword, and hound's head helm.