Susano'o is an irregular type of primal much like Odin. the kojin found 2 pieces out of 3 that were parts of Susano'o 's being you notice how he says "we" instead of "i" meaning that the 3 pieces are separate entities its exactly how zantetsuken is a part of odin an becomes whole when someone picks it up. the kojin being no strangers to Kami and the divine could hear the 3 pieces of Susano'o longing to be whole and so they tasked you the player with reuniting them however they were unawares that the pieces would summon a god, even the red kojin were confused and had just assumed Susano'o was a kami of their treasure hoard. when Susano'o was first summoned we don't know all we do know is that Susano'o is definitely primal-like as he divined that we were enemies to the red kojin and for their part in protecting its sword and mirror Susano'o offered to fight on their behalf even tho we involuntarily summoned it.