The manga opens with the shadowy image of a crying girl, and a tale of a child named Yuuko being sacrificed to protect the school. We then learn more from a vision that Teiichi has while talking with Yuuko: Yuuko was not only sealed up in a basement, but also had a broken leg - she died scared, alone and in pain, pleading for help all the while. The vision is somewhere between chilling and saddening.
Anime
Episode 4 gives us our first proper look at Shadow Yuuko as she stands over a sleeping Kirie, displaying a full on Slasher Smile full of dagger-like teeth.
Episode 6 re-introduces the Human Sacrifice plot point above. Yuuko had made up a monster that was terrorizing the school as part of a gambit that ends up more effective than she'd intended: she's offered up as a sacrifice to the monster, with people she thought of as friends willing to go through with it to save their own skin - and it wasn't even out of malice towards her...
Episode 10 is equal parts heart-rending and horrifying: Teiichi is trapped inside Yuuko's body via memory flashback and made to experience Yuuko's "sacrifice" - being thrown into a filthy pit and left there to slowly go insane and die of starvation, unable to move the entire time. Not long after, Teiichi decides to block her out of his mind and stopped seeing her, slowly forgetting about her before long. One is left to imagine just how traumatizing that was for him.
