The artist for some reason saved this image as JPEG with estimated quality of 41. If you zoom in slightly, you would see 8x8 pixel blocks with quite sharp, distinct boundaries.
I mean, it was teased months ago in the anniversary PV, among other things, so we knew Koyuki was in the event... and Koyuki is, in fact, Koyuki.
I'm pretty aware that it is just Koyuki in pajamas and nothing else "questionable" so to see the upcoming event is basically "Koyuki correction event" made it even more hilarious.
Saori blew a hole in Hieronymus the same way she blew a hole in Sensei
If you went up to this point in the story, the team Sensei summoned with his card are not the students that are helping him right now. Rather the students summoned by him is implied to be "his" students from another place (in game you're force to use your own students for this boss fight instead of using story characters, making this boss fight huge gatekeep for low level accounts) hence why this Saori is no longer wearing her mask and is perfectly okay with her face reveal.
It’s a reference to a scene from the ping pong animation, where one of the characters (Kong Wenge) sings ‘Hitori Bocchi no Christmas Eve’. You can see here at 1:02 https://youtu.be/8i93vXUKqnI?si=B0JyPaPttL5HXSrA
