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According to Wikipedia... Leah is the only female member of the La Push pack. She transformed into a werewolf during the events of New Moon, around the same time as her brother, Seth. This transformation is believed to be what caused the heart attack and death of their father, Harry.
She dated Sam Uley for years, until her cousin Emily came to visit and Sam imprinted on her. Sam thus left Leah for Emily. She believes that Sam betrayed her, but she still longs for him.
Leah is thus very bitter and cynical, and constantly antagonizes the pack by thinking about things that the other pack members want to avoid dwelling on, such as Embry's parentage. She also likes to antagonize Jacob by bringing up the fact that Bella is marrying a vampire and will shortly become one, and to get over her when it is clear that Jacob is still in love with Bella.