They can definitely implant false memories. When they hallow you out they have to put something back in or you won't function as a normal student. When it happened to me, I even had a best friend in class. Her name was Hitomi. She's a real NPC on campus, too. She still says hi to me even if I completely ignore her.

[ Yuri seems to think this over for a moment, but there are a few things that don't match up according to what she does know. ]

My problem with that hypothesis is no one's seen a shadow since you guys got here. The NPCs have been acting weird, but they haven't gone into that form lately. Plus, the idea of the shadows is to preserve people until conditions come that they would be capable of disappearing. Then the last thing... Well, it's more a matter of having to trust ourselves. If you're questioning their memories, they could just as easily say that yours are the ones that are wrong right back.