I really like how the Western Emperor was able to figure it out like that. I mean, he's supposed to be a genius among geniuses. As such, it makes a lot of sense that he figured it out once he knew that both Marie and Morina weren't well known despite Marie's supposed knight family background, that Marie was from Cloyan, AND Marie was extremely skilled, far more than a simple girl from a fallen knight's family aught to be. Out of all the people, I'm not surprised he figured it out first.