[Kiwami]

I rather think he is the last survivor of either the last world that was devoured before Helheim or any other world before then. As already said, he is called "snake" by the Overlords, maybe because he led them to their doom by telling them about the fruit, like he did with the humans of our world. The powers he possesses could be granted by the fruit, Kouta begins to develop similar powers like creating lockseeds in the same way as Sagara does.

He could be someone tied to the fruit, who has begun to see its way as a game. Maybe he is the very first one to hold it or even its creator.