魂魄 妖夢 is a half-human half-phantom who lives at Hakugyokurou in the Netherworld as a fencing instructor.
Youmu is a swordsperson of a two-sword school of fencing. She also had the responsibility for collecting and returning the phantoms that wanders around Gensokyo until some time after Undefined Fantastic Object, when the barrier of the Netherworld was repaired.
The longer sword, carried from her left shoulder to her waist on the right side, is the Roukanken (楼観剣, lit. "high-building sword"). It has the power to kill eleven phantoms in one slice. The shorter sword on her left waist is the Hakurouken (白楼剣, lit. "plain-building sword"), and it can slice through people's confusion.
She has a straightforward diligent personality. Luckily, she isn't manipulated by those around her a lot. Thanks to this, she often succeeds at her task. She also particularly excels at instantaneous force and concentration power.