* "Paige Matthews": Played by Rose McGowan, Paige is a half-whitelighter, half-witch who joins the Halliwell sisters in Season 4 after Prue's death. It's possible you're thinking of Paige and the name has been misremembered as "Patty".
* "Piper Halliwell": Played by Holly Marie Combs, Piper is the middle sister and has a powerful ability to freeze time. Maybe you're thinking of her name and it got shortened to "Patty" in your memory?
Please provide more context or details if you're thinking of a different character!