Think it's basically each of the students of that great mage ended up cursed some way, the one girl to look like a monster to everyone else and our master girl to see everyone else as monsters which is why she's so insane over the MC since he looks normal in this world of horrors for her. The MC seems to be immune to the curses so he gets seen as normal and sees everyone else normally making him seems like the last connection to reality to those two witches.