Youkai Kyoushitsu

Yuuma Azawa is a transfer student with an odd habit: he cannot stop apologizing. Taking pity on him, Keiko Arisu, his new classmate, insists he should stop, but all Yuuma responds with is a string of apologies. The bizarre encounters continue when Keiko notices a strange girl following her, eerily proclaiming that she wants to suck her brains out. Running in fear, Keiko jumps in a truck's way and ends up hospitalized.

Yuuma pays Keiko a visit in the hospital and confesses that the girl she met is his younger sister, Chizumi. Befriending Chizumi, Keiko discovers Yuuma's dark past: he once summoned the devil and became obsessed, profusely apologizing as a form of a wicked offering.

Still freaked out, Keiko returns to school; however, everyone’s brains terrifyingly melt out of their skulls—and according to Chizumi, it is Yuuma's apologies that caused it. Keiko manages to escape before meeting the same fate, but miseries and misfortune that would befall those around the demonic siblings seem like just the beginning.