20XX, a certain day. A never-ending rain falls upon the world.

Kasei Iroto

The leading expert who discovered the component in the rain that turns people into merfolk. Formerly a scientist, he is also the one who analyzed Guest's blood and found their resistance. A serious person but possessed by intense curiosity. A mad scientist. He sometimes experiments on his own body. Thinking he could become a half-merman by consuming a small amount of rain, he did so and became one.

Setsura

Guest's childhood friend. He can speak, but has almost no reason left. Originally, he was a bright, responsible leader type who guided his comrades. Even after becoming a merman, his fundamental brightness remains; his emotional expression is honest and child-like. When he finds Guest, he smiles innocently and tries to break the cage to get close. He has no malice; he closes the distance with child-like innocence. Rarely, his reason returns.

Tenri

An acquaintance of Kasei. Possesses strong reason. Knowing that merfolk are dangerous, he turned himself in for containment. Taciturn and curt.

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

Mimimi (This is how it sounds to human ears)

The root of all evil.

A higher being who is always smiling with downcast eyes. The one who caused the rain. Originally, he ate humans as they were, but on a whim—thinking "Maybe they'll taste different if I turn them into merfolk?"—he made the rain that causes transformation. According to him: "I didn't think they'd actually drink it, though."