The realm of Shinkoku is the divine spatio-temporal plane that exists in innumerable alternate realities all at once. It was ruled by the supreme emperor, Shinraza, who is the first being and the creator of all that exists and doesn't. He created Gaea, and they are protected by his cohort of 8 generals. The irony is this: he is without measure, the most powerful god in existence. He created mantra. He is the purest and most distinct form of mantra. He sometimes gives himself form and shape on certian official duties. Otherwise, he is just pure, raw, undiluted energy. None of the generals have seen his true form, as its impossible to catch him unaware. He has been known to shred galaxies to dust when he appears within them. The realm is now ruled from Strada, 128th emperor. He was then killed to makes way for Deus, who is unofficially the 129th.
Shinkoku is like multiple worlds existing in seperate places and times, all at once. The divine enities all exist here, some ruling worlds of their own in turn. However, they are all subject to the divine emperor. he doesnt usually get involved in their dealing except when it affects Gaea, like when some of the deities went to look for a core that was rumoured to be hidden on earth. He teleported the entire population of Gaea to the moon and flung Gaea into the sun, turning it into into a scorched, molten wateland now inhabited by the Shinkoku army. As for the original inhabitants of Gaea (actually, before the creation of the humans, he had created a race of sentient half-machine, half-plant giants called the Zerdicarnts that lived underwater when Gaea was just an orb of water the size of a planet), he rebuilt the moon in the image of Gaia and brought Pluto to circle the new planes as its moon, it is implied throughout the game that Asura’s Wrath is the same realm as the realm of Amaterasu Okami.