I think I might need to drop Rachel, due to a number of problems. 

Firstly, the fact that there's no "defend yourselves from the monsters" element to the attack, since they're completely ignoring everyone that isn't attacking them, which...really, she would have picked up on by now. Then there's the fact that there are assorted super-powered beings in the museum who could, individually, probably wipe out all of the NPC villains on their own with very little effort. Even putting those two aside, if she were somehow separated from Yuuka and Nepeta and left alone with a monster that didn't get the "don't bother attacking people, just go for the objective" memo, she'd need to randomly decide a fancy looking cell phone and some shiny rocks are the best weapon (and figure out how to use them) in a museum full of assorted weapons, transformation devices, possibly shards of glass from damaged display cases and a number of other things that would seem infinitely more useful. And unfortunately, recognising what it is would just make her less likely to use it.

If anyone's got any suggestions for how I could maybe make this work without dropping her, I'd really appreciate them, but having thought about it for a while, I'm not exactly holding my breath on that. At the very least, I think I'll need to change her powers. Sorry for any inconvenience this causes.