There's going to be a schism in Britannia, with world ending on one side, and the more important Britannian characters on the other. This other side will form an alliance of convenience with the order of the Black knights because the very world is at stake.
Expect Nunnally, Cornelia, Suzaku, Anya, Gino, Lloyd, and Cecille to be on this side. That's why they, Kallen, Toudou and the XingKe will all fight together at the end.
I'm not sure about Schneizel though.