ocean-moon nvfr anon was on to something
Furina (moon): reflects the light from the sun, has a dark side it hides from the ocean
Pre-split Furina (sun): the one which the moon reflects from
Neuvillette (ocean): the sun sustains its life and although the moon also reflects the sun, it has always been affected by the moon's true self, the moon's gravitational pull
(cherry-picked some astronomy stuff + i needed to convey this somehow)