Lumina doesn't look anything like Serah, and most Yeuls (except the nihilistic 500AF's) tend to have the same, quiet and gentle personality.
My guess? She's not human, Bhunivelze had Lindzei shapeshift into that form in order to monitor Lightning's activities.
Could be, my conjecture mostly came from the wonder why Serah even possessed the Eyes of Etro to begin with. And I guess given that Serah immediately died after Etro probably did.... Iunno, guess we'll see when the game's out. Pink hair, swords, changing outfits, Hope.
The official Penelo of the Duodecim Boards. "I'll be going too. Every good sky pirate has a partner, right?"