The following is my person dissertation on how I believe Primals work, based on what has been presented thus far and my own understanding of it. It is quite possibly off-base or even outright wrong, but the degree of which is likely only to be known once we get more information on Primals. However, since it is still using information learned through the MSQ all the way up to near the end (as well as what little tidbits I know of Coil), I will contain the whole thing in spoiler tags.
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Gegenji's Primal Thesis
A Primal, at its heart, is nothing more than belief and desire manifested. If you have enough aether, you can create a Primal of anything - from a dead hero to a mythological figure to a doll you found comfort in as a child. I specify aether because we have two requirements to summoning a Primal - crystals (solidified aether) and the belief of people. I posit that in applying belief, you are also giving up a bit of your aether to the Primal itself - just as one uses their own aether to affect the world around them in other aetheric ways (such as Thaumaturgy or Arcanima).
As such - and has been pretty clearly defined in the MSQ with regards to Shiva - even if someone summons a Primal of someone who once existed, it is not actually them. It is an aetheric simulacrum based on how the ones summoning it believe it should look and act, and the Primals in turn act based on these beliefs set at the point of summoning - a sort of "aetheric blueprint" set upon them wrought from the mind of the summoner. I would further suggest that Tempering is also an extension of that - wrought from the belief that the summoned Primal should not only be able to defeat one's enemies, but also to turn others to their cause. In this case, I would like to put forth the idea that Enkidu would actually not be able to Temper - because it's not a thing Gilgamesh would think his companion could do. Meanwhile, I believe Shiva most likely could - even if it was not utilized from what we saw - because she is seen as the champion of the Heretics and one to turn others to their cause.
This would mostly explain why the Primals we see summoned repeatedly have the same look and do the same things (beyond the obvious "it's a game mechanic" explanation, of course) - all that information is still fresh in the minds of those under thrall, and in the remnants left of them in Tempering. And so they they try again, using more crystals and more faith and more belief on what their Primal could or should be able to do to overcome their foes. I would figure, if a Primal fell out of "use" to the point that there was no visual representation to affix to it... its "resummoning" would look much different, to better fit the image in the mind of the new summoner. Thoradin and his Twelve looked that way when summoned because that is what the Pope and his men thought they should look, empowered with the faith of the Ishgardian people, just as Louisoix took on the form of a Phoenix for his Primal form - he saw it as a symbol of rebirth and appropriate to restore Eorzea, as Sounsyy has stated.
As such, I believe that there needs to be no physical basis for the summoned Primal. The hero being summoned does not ever have to have existed beyond in story or the mind of the summoner. If someone had an imaginary friend who only they could see, if they summoned it to prove it was there - it would work. It would take the appearance and mannerisms applied to it by the summoner.
Which is why constant aetheric power is needed to maintain a Primal - they are not native or natural to the world of Hydaelyn. They are like an extended spell, and require continual powering to avoid fizzling out and "ending." The bigger and more powerful the Primal, the more aether that is required - either through followers, crystals, or through directly drawing it from the planet in a manner not unlike Black Magic. Which is why Enkidu was hardly a blip on anyone's radar - likely able to be sustained through Gilgamesh's aether-fueled desire and the smallest amount of additional aether from land or crystal (or if at all) - and hulking Alexander threatened to drain the Hinterlands dry in a matter of weeks.
In the most general sense, if something is summoned in this manner - it is and always will be nothing more than a Primal. If someone summoned one of the Twelve - for example, Rhalgr as sought by the youths of Little Ala Mhigo - it would not be one of the gods at all but a physical aetheric manifestation of what the summoner thought they would be. One could also wonder if the Twelve actually exist at all to be called upon in any case, but that deviates from the topic and is worth its own discussion. However, it's still hard to say whether the Twelve that showed up to aid Louisoix were in fact the Twelve, or Primal manifestations that did what the wishers wanted them to do - empower Louisoix to stop Bahamut from laying waste to everything. The end result in either case was the Phoenix, a form wrought from aetheric power shaped by the wishes of Louisoix.
This is why, I believe, the Ascians haven't done the same to summon Zodiark despite having the knowledge to do so - they don't want a Primal of Zodiark. They want the ACTUAL Zodiark. And that requires much more than gathering up a bunch of crystals and really wishing their Dark God was here.
A Primal, at its heart, is nothing more than belief and desire manifested. If you have enough aether, you can create a Primal of anything - from a dead hero to a mythological figure to a doll you found comfort in as a child. I specify aether because we have two requirements to summoning a Primal - crystals (solidified aether) and the belief of people. I posit that in applying belief, you are also giving up a bit of your aether to the Primal itself - just as one uses their own aether to affect the world around them in other aetheric ways (such as Thaumaturgy or Arcanima).
As such - and has been pretty clearly defined in the MSQ with regards to Shiva - even if someone summons a Primal of someone who once existed, it is not actually them. It is an aetheric simulacrum based on how the ones summoning it believe it should look and act, and the Primals in turn act based on these beliefs set at the point of summoning - a sort of "aetheric blueprint" set upon them wrought from the mind of the summoner. I would further suggest that Tempering is also an extension of that - wrought from the belief that the summoned Primal should not only be able to defeat one's enemies, but also to turn others to their cause. In this case, I would like to put forth the idea that Enkidu would actually not be able to Temper - because it's not a thing Gilgamesh would think his companion could do. Meanwhile, I believe Shiva most likely could - even if it was not utilized from what we saw - because she is seen as the champion of the Heretics and one to turn others to their cause.
This would mostly explain why the Primals we see summoned repeatedly have the same look and do the same things (beyond the obvious "it's a game mechanic" explanation, of course) - all that information is still fresh in the minds of those under thrall, and in the remnants left of them in Tempering. And so they they try again, using more crystals and more faith and more belief on what their Primal could or should be able to do to overcome their foes. I would figure, if a Primal fell out of "use" to the point that there was no visual representation to affix to it... its "resummoning" would look much different, to better fit the image in the mind of the new summoner. Thoradin and his Twelve looked that way when summoned because that is what the Pope and his men thought they should look, empowered with the faith of the Ishgardian people, just as Louisoix took on the form of a Phoenix for his Primal form - he saw it as a symbol of rebirth and appropriate to restore Eorzea, as Sounsyy has stated.
As such, I believe that there needs to be no physical basis for the summoned Primal. The hero being summoned does not ever have to have existed beyond in story or the mind of the summoner. If someone had an imaginary friend who only they could see, if they summoned it to prove it was there - it would work. It would take the appearance and mannerisms applied to it by the summoner.
Which is why constant aetheric power is needed to maintain a Primal - they are not native or natural to the world of Hydaelyn. They are like an extended spell, and require continual powering to avoid fizzling out and "ending." The bigger and more powerful the Primal, the more aether that is required - either through followers, crystals, or through directly drawing it from the planet in a manner not unlike Black Magic. Which is why Enkidu was hardly a blip on anyone's radar - likely able to be sustained through Gilgamesh's aether-fueled desire and the smallest amount of additional aether from land or crystal (or if at all) - and hulking Alexander threatened to drain the Hinterlands dry in a matter of weeks.
In the most general sense, if something is summoned in this manner - it is and always will be nothing more than a Primal. If someone summoned one of the Twelve - for example, Rhalgr as sought by the youths of Little Ala Mhigo - it would not be one of the gods at all but a physical aetheric manifestation of what the summoner thought they would be. One could also wonder if the Twelve actually exist at all to be called upon in any case, but that deviates from the topic and is worth its own discussion. However, it's still hard to say whether the Twelve that showed up to aid Louisoix were in fact the Twelve, or Primal manifestations that did what the wishers wanted them to do - empower Louisoix to stop Bahamut from laying waste to everything. The end result in either case was the Phoenix, a form wrought from aetheric power shaped by the wishes of Louisoix.
This is why, I believe, the Ascians haven't done the same to summon Zodiark despite having the knowledge to do so - they don't want a Primal of Zodiark. They want the ACTUAL Zodiark. And that requires much more than gathering up a bunch of crystals and really wishing their Dark God was here.
~