Olivia was written to be so OP that if she dies there will be nobody else who can stand up to her, at least so far. I imagine if Rosemarie dies here the author would be forced to create some OP character to raise the stakes for Olivia, so it's actually the better outcome.
Rosemarie better not be made into said OP char to contrast Olivia for sake of drama due "desire for revenge" or some nonsense like that that makes her OP.
If that's done, we're doing to dumpster the series by shoving in a character all of sudden that is as OP as the MC, defeating attracting point of MC being OP, so we can drag out the story with months on end of chapters with "drama" and overly extended "close" fights which we know Olivia will win in the end but we're just going to pad out the length with powercreep and shounen fights.
This is the trap of alot of OP MCs. They (authors) feel forced to add equally OP antagonist at some point, but all it does it make the series generic, ruins the enjoyment of OP MC, and pads out the story length with forced drama.