Have you seen the Lightning Returns Gamescom trailer? If you have, you will know that Fang was fighting with Lightning in some parts. Fang will not be the only guest character in Lightning Returns: FF XIII, according to the latest interview. The game design director, Yuji Abe stated that Lightning will have more than just Fang accompanying her. These characters of course are only going to be AI, and not playable.
This might be some news fans would like to hear: Lightning Returns originally only took 20 hours or so to complete at first. According to Yuji Abe at gamescom, the game has undergone a change to go on much longer. One of the changes Yuji Abe explained was that it is now possible to interrupt the flow of time (which interrupts the 13-day apocalypse)
Yuji Abe now estimates that the game will take 50 hours at most for the first playthrough. It is important to note that however, it is still possible to complete it in 20 hours if you skip cutscenes and events, etc.
The interview also confirmed that at TGS in September a new character is confirmed to be shown.
Source: Novacrystallis
I am happy to see the game changed to 50 hours, and to see that Fang will not be the only guest accompanying Lightning. It might be something like Tales of Xillia though. Maybe 10-15 hours of that 50 are just subquests or something. It is still better than 20 hours though!