There is a certain relation, which is extremely astounding when you think about it, that is revealed in the very last quest of the game. Putting it on spoiler tags
Louisoux makes it very clear, with every word, that the Twelve can be summoned into the world the same way that one summons a Primal, and like a Primal they will eat copious amounts of Aether to take form. And then he sends you off to do exactly that, summon them to stop Dalamud.
Of course he has a reason to perform this risky move, supposedly the amount of Aether released from Dalamud's destruction would be greater than what the Twelve would consume, so it'd be a gamble, an even bigger disaster if it fails, great if it works