That was a beautifully executed twist. It called back the imagery of the first page. And it gives an actual reason why the aunt's appearance was hidden all this time. It brings so many things to a head. Sato's one friend being put to the test and failing, the boy being worried and running because he was worried about competition and being creepy again, the teacher being put in his place.
I really love this reveal. So glad it wasn't what we were all lowkey expecting it to be.
And we finally get to see Shio again. I was worried her absence was hinting at something, but I guess it was just the outfit change, phew.