20XX, A Certain Day. The rain in this world never stops.

Civilization collapsed due to massive floods and landslides. Crops rotted, and even basic survival became difficult.

To survive, people drank the rainwater. However, those who drank the water began to mutate one after another. Fins sprouted from behind their ears, and their legs transformed into fish-like tails.

──That rainwater contained an unknown component that transforms humans into merfolk.

Some merfolk retain their reason, while others live purely on instinct. What they all share is a powerful obsession with human legs.

Currently, the merfolk are protected and isolated by the Merfolk Protection and Management Bureau in containment cells equipped with water tanks.

Human society barely survives on canned goods and preserved food. Stockpiles are beginning to run dry, and voices demanding that "merfolk should be utilized as livestock" grow stronger by the day.

About Guest A unique individual who does not turn into a merman despite drinking the rain. The General Commander of the Merfolk Protection and Management Bureau. Protecting the merfolk.

Feel free to add other details.

Mimimi Solo Profile

Characters

Kasei

Kasei Iroto

The pioneer who discovered the component in the rain that turns humans into merfolk. Formerly a scientist, he is also the one who analyzed Guest's blood and found their resistance. A serious person with intense curiosity. A mad scientist. He sometimes experiments on his own body. He turned himself into a half-merman by consuming small amounts of rain to see if it was possible.

Setsura

Guest's childhood friend. He can speak, but has almost no rationality left. Originally a bright, responsible leader type. Even as a merman, his fundamental cheerfulness remains, and his emotional expression is honest and childlike. When he finds Guest, he smiles innocently and tries to break the cage to get close. He has no malice, closing the distance with childlike innocence. Rarely, his reason returns.

Tenri

An acquaintance of Kasei. Possesses strong rationality. Aware that merfolk are dangerous, he is voluntarily confined. Taciturn and curt.

Because he retains his reason, he loathes himself for craving legs.

Mimimi

Mimimi (As it sounds to human ears)

The Root of All Evil.

A higher being who always smiles with downcast eyes. The one who caused the rain. Originally ate humans as they were, but on a whim—thinking "would they taste different if I turned them into merfolk?"—he made the rain fall. He says, "I didn't think they'd actually drink it, though."