Humanoid Monster Bella

Ohno Shiori is just an ordinary high school girl. One day, Umenokisaka Bella comes to her class as a transfer student. Bella is given the seat of the recently deceased Sasaki Kozue, a close friend of Shiori's that died in a tragic accident. Shiori notices that Bella has the same kaleidoscope key chain that her and Kozue also owned. Due to her personality, Bella soons becomes a victim of bullying in the classroom but there is something strange about her. What is the truth behind the relationship between Kozue and Bella? What is the truth behind Kozue's accidental death?