We know that Ichirou (Satou) was married to Ame no mizuhana... I sincerely think that the creator God is Ame no Mizuhana, after all, the creator god came from another world, and she was there to stop Hiraru from returning.
The dragon god plays an important role, but surely it was he who summoned Satou at the request of Mizuhana-Hime... and continually gets involved in the world, had a fight with other gods, and taught how to summon Heroes.
Chapter 12 SS 2:
"This is the first time I've heard of Creator God--"
"That's only natural. It's a God that came from a far far away world together with the seven pillar gods after all."
Lua-san answered my doubt.
Come to think of it, the same thing was written in the picture book about myth from before.
"It's independent?"
"Yes, about 100 hundred million years have passed since they came to this world, but not even once an oracle came from Creator God. There are many gods who leave their worlds, and that particular God has non-interference policy."
Chapter 10-49:
"God, are you inhibiting civilization like the demon lord's said?"
"I have no interest in people's business, at least for me. My only interest is only you always."
Now, we know that Satou has his memory blocked, but it is a "desire" of him, when Satou met with Menea he was seduced by her in a very natural way, in turn, in the castle he was not attacked until he defended to Hikaru, so he must have some kind of relationship with that castle, however he was attacked by the Apostle, if the apostle served Satou he would have recognized him at least.
Ichirou has been continuously reincarnating, because he needs many souls to ascend to the divinity, it is possible that this Ichirou, is the Ichirou(Satou) with all the souls of his reincarnations and parallel worlds.
If she has always been protecting Satou I don't see anyway of her becoming the Demon God, and even when Satou uses her unique abilities, he does not present a burden, he can use them unlimitedly even to go to other parallel worlds.