折り鶴に宿っていた何かが、長い年月を経て異形化したとされる存在。

大人しく、川や湿地を漂うようにゆっくりと移動する。

かつては小さな折り鶴に過ぎなかったが、現在では著しく巨大化・異形化している。白い紙のような身体と独立した頭部を持ち、それらは紐のようなものによって繋がれている。

過去に、無数の折り鶴がまるで命を得たかのように動き出す現象が発生したと記録されている。その後、その多くは時とともに朽ち果て、原形を失った。しかしごくわずかな個体は現代まで残存しており、紲羽はそうした生存個体が長い年月の中で変化を続けた姿であると考えられている。

An aberration believed to have originated from something that once resided within origami cranes, gradually transforming over the course of many years.

Generally docile in nature, Setsuu drift slowly through rivers and wetlands as though carried by the current.

Though they were once no more than small origami cranes, they have since undergone significant growth and transformation. They possess bodies resembling white paper and a detached head, the two connected by cord-like strands. The exact nature and purpose of these strands remain unknown.

Historical records describe an event in which countless origami cranes began to move as though they had been imbued with life. Over time, most of them deteriorated and lost their original forms. However, a small number are believed to have survived into the present day. The entities now known as Setsuu are thought to be among these rare survivors, having continued to change and grow increasingly aberrant over the centuries.