Youmu is a half-human half-ghost, who lives in the Netherworld, the land of ghosts in Gensokyo. Even though she is a cross-breed her two sides aren't actually joined together, in fact her ghost-half follows her around as a separate entity. Youmu is a very straightforward and focused personality, which helps with her swordsmanship, but conflicts with her mistress, Yuyuko, who is more playful and ditzy in nature. She's actually supposed to be Yuyuko's swordsmanship trainer, but she's more of a gardener and a personal guard to her.
Youmu's predecessor and teacher was Youki Konpaku, who after serving Yuyuko's family for 300 years suddenly decided to become a hermit, leaving the very young Youmu as his successor. As Youmu's swordsmanship training was still incomplete, she was left to train herself, which she does between patrolling the gardens.
Even half-trained, however, she is still a formidable swordswoman. Her technique uses two swords. The longer one is called Roukanken and it has the power to cut through ten ghosts at once. The shorter one, Hakurouken, on the other hand cuts through a person's confusion. Of course, both of them can also be used for the traditional purpose of just cutting up people. In fact the things they can't cut through are next to none.
Perfect Cherry Blossom
During the events of Perfect Cherry Blossom, Youmu is asked to bring the spring's essence from Gensokyo to the Netherworld, so Yuyuko can have her cherry blossom garden bloom perfectly and discover who was buried under a certain youkai tree. As this causes winter to continue indefinitely in Gensokyo it puts her face to face with the heroine of the game. Youmu first tries to stop her from getting to the Netherworld and later to Yuyuko, but she is defeated both times.