So, Luna is a bratty girl, Sena is a kind boy, and Platinum is...Platinum. The picture doesn't look bratty at all, so maybe she defaults to Platinum? Maybe Luna and Sena only come out in story and not gameplay?
Also, Luna has a habit of referring to herself in the third person, though Sena refers to himself in the first person, as does Trinity.
As well, apparently Trinity herself resides in the Nox Nyctores, Muchourin. Sena and Luna share the body itself. And during one of the story mode paths, the staff that Platinum carries is also called a Nox Nyctores (they never give it a name, just say that it is a Nox Nyctores). So, they have 2? It could substantiate my thoughts that the staff serves as nothing more than a hammer for the bell.