Youmu is one of the stranger entities in Gensokyo, part-human and part-ghost at the same time. Her two halves are not intermixed like one would expect from a cross-breed, but actually exist in physically seperate bodies. Youmu has a straight-forward and focused personality, which complements her swordsmanship abilities well. However, her personality conflicts with that of her master, Yuyuko Saigyouji, who is very whimsical and playful in nature. Still, she serves Yuyuko as best as she can, tending to the gardens in Hakugyokurou and following her orders.

Youmu commonly wields two blades at once, having studied some under her father, Youki. The longer blade, called 楼観剣 (Roukanken) - Tower-Watching Sword, is said to be able to kill ten ghosts in one swipe; the shorter blade, 白楼剣 (Hakurouken) - White Tower Sword, is able to cleave through a person's own confusion. However, not much is known about her particular style of swordsmanship, or her parents. What is known is that Youki, also a servant of Hakugyokurou, had begun training a very young Youmu (possibly as a child?) when he suddenly disappeared one day, leaving her style incomplete (and presumably causing her to teach herself). No one currently knows his whereabouts.

(Source: TouhouWiki)