There had already been some rumours, but now CD Projekt Red haveconfirmed, that Ciri is the second playable character in The Witcher 3: Wild Hunt (teased at the Game Awards). But the Polish developer also announced, that you won't be switching between the two on the fly, but you gain control of Ciri for a short while in certain moments, which will be very significant for the story and should make it more engaging.
So she is not a real second playable character next to Geralt, but a supportive narrative device, to let us experience the game from a different perspective. She is emotionally bound to the witcher because she knows him since she was a child, but she has an agenda of her own as well.