I think the plot is leading up to where kay does get possessed by the goddess of darkness but then she realizes that Aida is the reborn light goddess and the reason she doesn't have magic is cause she gave all her magic up.
Omg, imagine if that was their first life, then they reincarnated on earth and become friends, without all the magic to ruin their “friendship” and then now they’ve been reincarnated again into their original world as their reincarnations. So they actually are the original 2 goddesses!
I think the plot is leading up to where kay does get possessed by the goddess of darkness but then she realizes that Aida is the reborn light goddess and the reason she doesn't have magic is cause she gave all her magic up.