Originally Posted by
Lily Emilio
Elementals are all closest to the planet out of everything it created, especially those who are born from the inner sea. However they have their "role" being determined since birth to do specific jobs the planet wanted them to do with. Being the brain, with new implications, seems to be yet another one of the jobs that unless the planet specifically choose you, no one can get that role. And the job seems to involve being the direct administrator of the planet, resulting from the whole asking Crimson Moon to help with the rise of humans. Basically prior to Crimson Moon, it seems that the idea of having a "brain" like that never occurs to Earth as a living celestial body, as the Elementals it created were sufficient enough to act as terminals to manage the surface (like Yu Meiren), which was why it never got a Type like other planets, until humans became a threat that they overthrown the previous Prime Species - the gods. The furry A-rei in Lostbelt 6 for example, has the same "Breath of the Planet" skill as Arcueid, Yu Meiren has even better regeneration than prob most TAs (girl literally blows herself up into a rain of blood and instantly reconstruct herself as will, no vampiric impulses, can pull nigh infinite mana from the world, spirit core is one with nature...etc), and Arcueid is now called a "True" type of A-rei, who looked at Excalibur and believe it to be sorta her "sister", referencing the six A-reis whose job was to forge Excalibur with their body to fend off Sefar.
Another point is, it was mentioned in Olympus LB that powerful chief gods of some pantheons such as Zeus are capable of controlling most natural phenomena, making them the "closest to being the ruler of the planet", but they are not Ultimate Ones. The Lostbelt race at the beginning is also to "find out the King of the new planet". Kiara as Beast III was trying to make the planet her body (so she is pretty much gonna become its brain). Goetia tries to become the foundation of a new Earth he plans to create. In Extra, it was also said that the Moon Cell after evolving to the point it can affect the planet with its function beyond everyone else, is called "administrator of the planet", dubbed as the "Type-Moon" of Extra. The goddess Tiamat's appearance embodies the planet, with the horns representing the continents, the hair is the sea, her eyes reflects the inner sea of the planet, described as a perfect being with no physical and mythical flaws, when she awakens she caused spacetime drifts similar to when Archetype awakens in the MBBAAN manga, has no death concept like Types and has similar vibes to Type-Venus. And yet, none of these entities are candidates for Types, the planet didn't go ask them hey can we make some deals. So to me it implies that entities having aspects similar to an Ultimate One can exist, as in, governing the planet itself through their crazy powers or embodies the planet, but would not get recognition by the planet since they are not as "close" to the planet as the Elementals, especially ones born from the Inner Sea, so they are always lower rank than the planet's agents, and thus can never be the One. I think the main antagonist of Lostbelt arc is trying to get that role by force, just like Goetia and Kiara tried to do.
So with all the new lores, it seems that the planet created TAs the same way they created these high class Elementals i.e the A-reis in the past. But their intended purpose is to be the brain rather than doing different things. Pretty much instead of multiple Elementals doing different functions, one individual who can do ALL functions is the new idea. In other words, there is a pool of Elementals who are not much diff from the TAs in term of their being, but it is because of the fact that their role cannot be changed, there is no "ascension" to become the Ultimate One, the planet had to create the TAs to fulfill that role after consulting Crimson Moon. And among the TAs, it then have to pick between the most perfect ones, namely between Arc and Altrouge.
That being said, I think it works a bit differently in Fate timelines, due to the downfall of the DAAs, Zelretch not being vampiric and Roa was toasted by Edmond Dantes. We don't know if Altrouge exists in Fate, because PM lives with Merlin now. But if she does not exist, Arc should be the default "brain" in Fate timelines imo.