Youmu is one of the stranger entities in Gensokyo, part-human and part-phantom at the same time. Her two halves are not intermixed like one would expect from a cross-breed, but actually exist in physically separate bodies. Youmu has a straightforward 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.
Like the majority of the Touhou characters, Youmu's involvement in the RP was minimal at best. She debuted on May 10, 2009 (the same day as her mistress, Yuyuko). When Yuyuko was chasing Luvbi for a little snack, Youmu stepped in when Luvbi somehow managed to fight back. Luvbi begged Youmu to make it clear that she meant Yuyuko no harm, and thank goodness that Youmu actually listened.
Much later on (around mid-November), Youmu came across Amami Serizawa. The half-ghost made the Kuchisake-onna sign a contract that disallowed her from cutting up people's mouths unless it was out of self-defense. The shrine maiden Reimu Hakurei had also heard about the threat that Amami had caused and tried to exterminate her without realizing the contract Amami was under. Youmu discovered Reimu's actions and scolded her for them.