Katherine was her father's daughter through and through. Think of the words you would use to describe Lord Walder Frey: vain, ambitious, untrustworthy - all could equally describe Katherine. The Frey family was huge, his father had married eight wives, had twenty-two true born sons and seven true born daughters, not including Katherine herself. It would have been simple to become lost among all those faces, and at times Katherine did, but only when it pleased her, as she wanted to be known.
The only child of his fifth marriage and no full siblings to call her own, perhaps it was loneliness that drove her, to fill that aching void; there had been the stories of a still born sister after all, a twin that had been identical except that Katherine breathed while the other girl did not. Perhaps that story put herself aside from the others. Not enough though.
She was the most beautiful Frey daughter, but the most wild. Her father had been keen to marry her off quickly; perhaps a babe in her womb and husband in her bed would quieten her down, but that dream turned to ruin after she bedded one of the Knights of The Twins. But she was cunning too, when she was done all believed her to be an innocent lamb and while it was her that had taken the Knight by the hand and lead him to bed, it was quickly believed the reverse and none would ever dream of doubting her word. And after that, she knew how to keep all of her misdoing hidden.
She enjoyed the trips to King Landing, the silk's were finer as were the company she could keep; there was one boy she remembered with more fondness that the others, perhaps it was because he was not as interested in lay between her thighs but rather the words she spoke. He didn't bribe her with jewels as other men did, he gave her books and the occasional sweet smelling flower. Fondness, yes. But that was depth that Katherine dared to venture.
Of course, the happy life she had crafted wouldn't last forever. Eventually he father found her a match. She was married to Robb Stark, no matter how much she protested, and as the war between the North and South began, there were no more fond trips to the South and with a wedding band on her finger, she could bring no new men to her bed. But Katherine had power, and she rather liked the sound of that.