You're referring to "Haitoku no Kyoukai"!
The border wall was invisible, but everyone knew where it stood. Haitoku no Kyoukai
Haitoku no Kyoukai follows protagonists confronted with moral transgressions that blur the boundary between right and wrong. The narrative centers on a character (or small ensemble) who becomes entangled in ethically ambiguous decisions—romantic betrayals, crimes of passion, institutional corruption, or supernatural pacts—that force characters to confront personal culpability, societal taboos, and the consequences of surrendering to forbidden desires. The plot typically progresses from an inciting ethical breach, through escalating dilemmas and interpersonal conflict, to a climax where moral lines are decisively crossed or redefined, concluding with resolution, punishment, or ambiguous ambiguity. You're referring to "Haitoku no Kyoukai"
Through Shiori's character, the series highlights how individuals can lead seemingly normal lives while hiding dark secrets. This dichotomy challenges viewers to consider the complexity of human personalities and the masks people wear in their daily lives. The narrative centers on a character (or small