All of Westaros knew of the Clegane brothers ferocity and prowess in battle. There were many reasons for the younger brother Sandor to hate the elder, Gregor. However the major reasons were Gregors murder of their sister, the burns he sent across Sandors face, the killing of their father, and the treatment of young Ataya. Gregors daughter who is now motherless thanks to Gregor.
Ataya is the child that Gregor had with his first wife, the woman he came the closest to caring about. However his anger is never appeased and he killed her after she disobeyed him by writing to her family about the child. The same night he tried to kill the three month old girl, but was stopped by Sandor who took the girl back to Kings Landing with him. There was a valued lady there who had lost her child, and having Ataya around helped her to find reason to live again.
Sandor helped raise the girl, and she saw him more like a father/older brother than her real father would ever be. When people asked about her they were told the truth about her parentage and congratulated Sandor on his quick action in saving her. They agreed with most of the ways she grew up among the other young ladies sewing and learning how to keep a house. However Sandor also insisted on teaching her to defend herself, something that she soaked up knowing that this was more vital than being able to do a perfect stitch or set a perfect table.
When the Hand's tournament happened in the North, Ataya went as one of the maids. At fifteen almost sixteen she was pretty with long black hair and her mothers bright green eyes. Like her father and uncle she was tall, but slender and moved with grace.
When the Starks first noticed her, it was when Sandor lifted her from the carriage she was traveling in. She asked him a question and he put his hand on her shoulder before answering. A beautiful smile lit up her face and she matched his stride as they strode away. Seeing them watch Tyrion Lanaster spoke, "The Hound and his niece."
Robb looked at him, "His niece?"
Tyrion nodded, "Yes, the Mountains daughter poor girl." Seeing another question coming, "Her father killed her mother in a rage when she was only a few months old and almost killed her before the Hound stepped in. He's basically raised her since." Smiling slightly at the shock, "Not even hounds will abandon a pup." Then walked away to let them process it.