I was just playing through arcade with Rachel and Momiji and it got me worried that they might not be in Doa6.
Actually, I always wondered, why everyone seems to take it already for granted, that Momiji, Rachel, Marie Rose, Phase 4 and now Nyo Tengu will make it into DOA6. They might not (expect Phase 4 maybe)! Or they might not make it in at first, but again being DLC characters with no part in the story. I'm sure, we will see at least 3 completely new characters in DOA6 with their part in the story, like in every new game, but before mentioned characters might just be characters, that they put in, to sell DOA5U and DOA5Arcade.
In other fighting games it's completely normal, that some characters, even if important or popular, won't make it into the new game for some reason and the DOA roster of characters has gotten really big by now...