@danbruno ohhhh haha I only beat the true final boss. Which btw was really weird structure-wise with how they handled the whole thing? I almost wonder if it's a commentary on the "possible worlds" that Campanella mentions.
The consensus among fans seems to be that Grandmaster is Aidios, yes. Although she says she's like a minor echo of herself. I guess this means we will eventually fight the main church's god like in all JRPGs.
@danbruno aside: the English voice actor for Gaius plays the main character in Yakuza: Like A Dragon, which I'm currently playing and it's really throwing me