I feel like people are complaining about how Orstead switched gears so fast, but we don't actually know what the girl said to him to make him do that. She might have had a really good, convincing argument. We don't know, because Rudeus passed out, and Eris doesn't pay attention to details.
He's not a god-god. He's one of the Seven Sword Gods. Which is just a fancy name for really strong warriors.
He's presumably the 2nd strongest warrior in the world.