I'm thinking it'd be a great subversion if Pearl got the villain treatment instead of Marina. I mean, everybody expects the Octarian and there'd be more crunchy justification behind it, but it would undermine her development a bit if that relapse isn't handled with care. Meanwhile there are so many cool directions you could take a vs. Pearl scenario because she's already a lil' gremlin who'd be perfectly willing to throw hands given sufficient justification. Not evil by any stretch, but a burst of zealous anger or a gaslit misunderstanding or even a flat-out ideological schism would be enough to get at least a good boss battle. Also less of her character would be "devoted" to said defining moment than with Marina, so it'd still have plenty of room to breathe apart from it.
Does that make sense?