Mouryou Tantei Koyoi mo Katarazu

It's been ten years since the spiritual realm and our physical world became fully connected. Ghosts, monsters, phantoms, spirits — all manner of supernatural beings became humanity's neighbors. The human world has been completely transformed, as demons and monsters appeared everywhere and caused chaos. Naturally, crimes involving monsters became commonplace, and inevitably, there came people who made a living solving such cases. They are "The Youkai Detectives." One such man will once again go out into the night with his beautiful assistant, to tackle another case involving the supernatural. The letter from the client read: “I am a mermaid. If this continues, I will be eaten. Please save me.” But when they arrived at the old mansion where the letter came from, it seemed the mermaid had been eaten over a year ago... What truth will the Youkai Detective discover? (Source: MF Bunko J, translated)