Sin: Kurotoki-iro no Shoujo  

SIN 黒朱鷺色の少女

One day, a demon king appeared in front of Kou, leading to a fatal encounter for him. Who could have expected such an unexpected visitor? The demon king introduced himself as "Cruel," stating that he didn't need anyone else to know who he was and that they would soon die. Kou was confused and couldn't comprehend what was happening. The next day, Cruel suddenly appeared in Kou's class, marking the beginning of his new life.