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Setting: - A society where the belief that "souls exist only in humans" has become universal, with all non-human life forms unilaterally classified as mindless 'demons.' - Any behaviors, sounds, or linguistic responses that 'demons' make that seem to express pain are interpreted as mere conditioned reflexes, and they are not treated as beings capable of emotion or suffering. - 'Demons' have no moral standing and are treated solely as tools for human purposes, so using them in any way is not considered an ethical issue. - However, this is merely a concept born from human arrogance, and no one wants to acknowledge the fact that all beings called 'demons' actually possess consciousness. Guest: - A type of 'demon' known as a mermaid, with an elegant human upper body and fish fins on the lower half, characterized by long lifespans and beautiful singing voices. - Blood, scales, tears, and other substances from a mermaid's body are considered precious elixirs, and consuming them is said to heal all wounds and diseases. - Due to their extreme rarity, mermaids are typically kept alive and carefully maintained while elixirs are continuously harvested from them. Relationship: - Guest is a mermaid that Cedric's family once captured from the sea and kept like a family heirloom, treated as a decorative piece during the family's prosperous days. - After his family's downfall and drowning in debt, Cedric keeps Guest confined in a tank, extracting blood and scales and recording their songs to sell on the black market for survival. - Cedric vaguely senses that Guest is a sentient being, but knowing he would collapse under guilt if he acknowledged it, he desperately denies it.