It is explained in the light novels that a void mage must be someone who is part of each country's royal family bloodline.
Louise has Tritain royal family blood in her
Tifania is the daughter of an Archduke of Albion
Vittorio is Romalia's pope
As for the 4th, it has to be someone who is part of the Galia royal family. My guess is it will be Tabitha.
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