Mienai Kao

One day, Onozuka Miki receives a call from her older sister, Tatebayashi Midori, from whom she was separated years ago. Midori desperately begs for her help, afraid that a killer is after her life! Miki quickly transfers schools to Midori's posh private boarding school, but not two days pass before she is swept into a whirlwind of unexplainable murders...

Note: Included in vol. 2 is a one-shot by Matsumoto Yoko-sensei titled "Nemesis no Yoru" about a group of students who spend 5 days on a deserted island with a killer who murders them off one by one...