Uh, hm. Good point to make. I'm inclined to think that Platium is a girl considering the harem picture with Ragna, but if they were going to try a pull fast one like they did with Bridget, Platium would be their chance.
According to wiki she has three personas, one of which is a boy. I do not know if this means she/he literally changes sex when one person or another pops up, but it's a bit disgustng. I hope Luna isn't too broken a character, so that I can pound the crap out of him/her on-line.
I don't think spoiler tags are nessisary for this since it's just speculation, but uh, feel free to shun me or something if need be:
I'd imagine it's probably going to work in the exact same way it works for people with dissociative identity disorder; they are still their respective genders physically, but their mental gender, mannerisms and potentially even tone of voice will switch to suit the current personality. But, really? Who knows - they might make them physically manifest instead (be it Platinum herself or something akin to Dizzy's wings.)