She's songwriter Kara DioGuardi.
The Fox announcement pointedly says she will be joining Simon, Randy AND Paula. But with more than enough time taken up by three judges' comments, I have to wonder how long it will be before they either drop another judge entirely -- whether Paula or Randy -- or they use a rotation of judges, with three chairs filled by a selection from the four. (Sounds as if Paula is hearing footsteps.
Am skeptical of how well the four-judge system would work. When they have included a celeb in the fourth chair, it has not for the most part been a rousing success, since the celebs have for the most part proven reluctant to criticize. (The mentor system has the same problem.) And the last attempt at a fourth judge -- Angie Martinez -- was extremely short-lived.
But it's interesting that this announcement comes after Nigel Lythgoe announced he's stepping back from the show.