Forgive me, I forget sometimes that not everyone has access to a magic encyclopedia brain...Fi is the evil banishing spirit of the Master Sword. The fact that she isn't here in this weapon truly makes me believe that the cycle not being fulfilled as usual forces us to accept that there's a reason for it. Fi has never left her home before. And what's especially startling about this development is that no evil could remove her from it. Her spirit was imbued by the goddess Farore. She either would have had to leave the weapon herself, which she was sworn never to do without the goddesses approval, or someone with the power of a god is the cause of all this. To which I would immediately figure Demise.