Urumi Ushizaki is a young ushi-oni who has claimed part of the Sanzu River as her own. She makes a living by selling the prehistoric fish she catches in Gensokyo.

In the past, she would use her ability to change the weight of everyday objects to make her prey heavier, causing them to sink, before launching an attack, but not anymore. Since attacking humans was banned in Gensokyo, there have been no reports of her doing so. In fact, she's been known to help lost travelers who attempt to cross the Sanzu River. Her current friendly nature makes her dark past seem almost unbelievable.

The Sanzu River flows on the border of Gensokyo, with the Gensokyo side being Shigan and the other being Higan. The river separates the two worlds, and so it follows that none who cross it ever return alive. The fish that swim there are crudely referred to as "prehistoric fish," but they cannot be caught with any usual fishing methods. Perhaps because they're phantoms...

With Urumi having changed her ways, there are many who mock her, saying she's gone soft. Urumi is surely aware of this, but it doesn't seem to bother her, which goes to show just how content she is with her current way of life. But one mustn't forget no matter how she appears, the threat she poses will never fade.

The Sanzu River contains its own ecosystem. However, on that day and at that moment, the ecosystem was no more. The divide between hunter and hunted vanished, and the shadows of all creatures morphed into those of hunters. Heeding to their master's will, all descended upon a single point, accompanied by a wild, thrashing torrent. The Sanzu River moved as one and bore its ferocious teeth.