Dark Cat(1987)

Dark Cat

A school monster horror depicting the activities of Kagezaki Hyoi, a dark cat who defeats the "demons" that possess humans, and his younger brother Rui. Aimi, a junior high school student who tends to be absent due to asthma, calls out to her childhood friend, Hiroki, when she goes to school, but he is cold. And during cleaning time, transfer student Kagezaki Shimoi approached the White Snake Mound, which is rumored to be ominous, and found a skeletal corpse...!?

(Source: eBookJapan Plus, translated)