Mong Jung Mong (몽중몽)/theories

Lee Jin Hui's Family and her Amnesia
Three special NPCs who we'll be calling Helper/Ani, Friendly Person/Gidae and Doctor/Gyosu (seen in the lab of the Non-destroyed town) might be members of her former family. Gidae led Jin Hui throughout her childhood and Gyosu was some sort of scientist. Judging by this info and fact that Gidae and Gyosu have the same hair color as Jin Hui's, they could be her parents or cousins. The reason why she doesn't remember anyone of them, especially Gidae's face is Jin Hui got some form of amnesia during the apocalyptic event. (possibly the "Certain Incident" mentioned in the text after the "Go Outside" event)

This could explain why the gray-haired NPC had no face and why students in her school had to distinct features, as well as why the red-haired girl remained in her silhouette form until Jin Hui used her School uniform effect.