In one of the forests of Gensokyo lived a young youkai of darkness named Rumia. The short blonde-haired, red-eyed youkai had a somewhat petite figure similar to that of a 10-year-old girl, and she wore a black vest and skirt snd a white blouse with a red tie around her neck. She also wore a red amulet in her hair like a ribbon, and she couldn't remove said amulet as it was sealing a greater power from her, according to the rumors from some people. As a youkai of darkness, Rumia has a perpetual sphere of darkness surrounding her body, and although Rumia can control and manipulate the darkness around her, said veil of darkness causes her to fly around aimlessly and bump into obstacles due tithe darkness obstructing her vision.
Rumia lives a simple life of a youkai, preying on humans to kill and devour. Although humans native to Gensokyo are off limits to be eaten by youkai, humans from the Outside World who wandered or were "gapped" into Gensokyo are fair game. Rumia was a voracious youkai, and there is nothing tastier to Rumia than the flesh of a human. Outsider humans were especially tasty to Rumia, and Rumia could tell whether an Outsider human was nearby through their scent. Lately, more Outsider humans have appeared in the forests of Gensokyo, and very few were able to escape being eaten by youkai. Very peculiar was that some of the Outsiders were tied up and wearing nothing but a fundoshi and a sack with the kanji symbol for 'sin' over their heads. Probably they were criminals and their punishment was to be hunted by youkai? At least Rumia found those types of Outsiders, dubbed 'Sinsacks', much easier to hunt down; Rumia was getting lazier with her hunting and found it generally a pain to do, especially when chasing humans had gotten her running into more trees than humans. Rumia's lifestyle of hunting and eating humans was catching up to her, though, as Rumia was developing a slight pudge on her body.
When Rumia is not hunting humans, she is seen hanging out with her fellow youkai friends. They include Cirno, the ice fairy who lives at the nearby Misty Lake; Mystia Lorelei, the night sparrow youkai who can sing and owns a grilled lamprey stand; and Wriggle Nightbug, the firefly youkai who can control bugs. Rumia were great friends with them, but for some reason she couldn't help but think about having her friends inside her, whatever that means.
One day, Rumia was flying in her forest searching for humans to prey on. Suddenly, Rumia: