Abyss Azure No ZaininAbyss Azure no Zainin; アビスアジュールの罪人; Abyss Azure no Tsumibito; Грешник лазурной бездны; 碧蓝深渊的罪人; Sinner of the Azure Abyss; A Sinner of the Deep Sea

For many centuries, merfolk have lived in the depths of the ocean—4,000 meters below the surface—where humanity cannot find them. However, one mermaid's life is turned upside-down when her best friend falls for a human.