Spoiler in the afterword of Volume 4

The author gives away a spoiler there, not that is much of a secret since he makes it really obvious in volume 5, he's been dropping hints since volume 1 so... what I find weird is the fact that Arcus doesn't catch on given that he's supposedly really smart, he's truly an oblivious protagonist

It's a pretty open secret in-lore too since it seems everyone but Arcus knows about it.

The spoiler
Sue is Ceylan
Tbf as of Volume 5 is unconfirmed

Edit: so let's have a meaningful discussion on it, eh? Wouldn't want this very visible thread to get removed