Lumina will have to be a relative or something. She looks way too much like Serah, and that hair...seriously, no one else in the FFXIII universe has that hair. It's totally a signature hereditary trait of the Farron family.

Or maybe they're keeping good on that whole "Lighting is female Cloud" thing and pulling an FFVII-style clustersmurf with the story. It's not like they can't do whatever they want at this point with how XIII-2 ended.