Stupid Twisted wonderland Theory incoming:
So, Yuu is gender neutral in the game right? And in the manga Yuus gender changes (book 1: male, book 2 female)
So a girl/nonbee in all boys school? Kinda confusing right, but on my theory possible!
The magic mirror says that our soul has no colour or shape, cause yeah. Probably because we are from a diffrent world, so in my theory the reason the mirror is not able to tell anymore who we are and why we were called here is because: Our soul is not in its "right place"!
The mirror can be seen as a portal and yeah, so in my idea is that the mirror is actually able to see through worlds, but once someone travels through that world their soul is out of place and kinda colour less