Kokodake no Himitsu(2013)

Kokodake no Himitsu

Mari Kurusu finally gathers up the courage to confess to her crush, Kuro Rouzen. However, instead of reading the lover letter, Kuro eats it instead. Hearing it may be because he has a girlfriend already, she stalks him after school only to find him battling a beast... and having tentacles come out of his head...