Siliconera had a nice chat with the Director of Final Fantasy XV, Tetsuya Nomura.
According to Tetsuya Nomura, Square-Enix has decided to make Lightning aka Claire Farron, apart of Final Fantasy XV.
Siliconera: "Will she be a playable character?"
Tetsuya: "She will not be a playable character, but instead she will help you in the world of Final Fantasy XV in other ways. She will be able to be called upon at random to help you in battle, kinda like Mickey was for Kingdom Hearts: Chain of Memories. At random points there will be a trigger button that you can press and she will join you in battle for a short time."
Siliconera: " Will she play any real role in the Final Fantasy XV story?"
Tetsuya: "Yes, she will have a major plot in the story, we decided this before Final Fantasy XIII: Lightning Returns was released, but all of her memories will be gone from the XIII story, and she will go by Claire Farron.
Siliconera: "Any other things you can tell us about her?"
Tetsuya: "Only that she will be able to use many of her abilities from Lighting Returns, depending on when you get her."
Guess we will get more Lightning afterall.
[Source: Siliconera]