Theory time yay~
Looking at the cover art, I think it's either a yama or an oni. She seems to be holding a yama stick and a judgment scale. There also seems to be a sword hung around her hips. I think some yama have horns. It could also be the third oni deva, wielding a couple of weapons. Or it's an instrument, like a big violin. Layla perhaps?
I have a feeling this would be the extra boss instead of final boss. A yama trying to put a stop to these necromantic shenanigans but arriving a bit too late. Or it's the phantasm boss. There could still be a chance for Mimerz or Layla to pop up.
I'm still hoping that the final boss is a revived Qin Shi Huang, the first Chinese emperor, with his terracotta army as stage enemies. Would explain why Yoshika, a jiang shi of all things, appeared in this game. Likely Qin Shi Huang will be turned into a loli ghost, but it'd be really cool if ZUN made him male. Even cooler if instead of a pretty boy it's a freaky undead skeleton or lich. It'd certainly be something different.