A carnivorous youkai with the ability to manipulate darkness when attacking humans. She dislikes sunlight, concealing herself in darkness to protect herself, but ends up impairing her own vision, leading her to be portrayed as a member of Team 9, a posse of childish low-tier Touhou characters.
She is best known for her catchphrase "Is that so~?" (そーなのかー?, "So-nanoka~?"). The Pixiv tag 毎月七日はルーミアの日 ("Maitsuki nanoka ha Rumia no hi" / "The seventh of every month is Rumia day") is a pun off of her catchphrase, since "nanoka" can also mean "七日" ("seventh day"). She is often drawn with her arms spread in a "crucifixion" pose.
On occasion, she is depicted completely surrounded by darkness, flying around as a solid black sphere with only the presence of her catchphrase or her ribbon sticking out from the darkness identifying her.
