Rumia is one of the main characters of the Północ Projekt series.
She only roams around the night, and also travels in a mass of darkness. She is a very strong youkai that can manipulate the darkness, and can shut off power in towns.
Also as seen from the games, she likes to travel around towns aimlessly. In the fourth Północ Projekt game, she also mentioned that she made a few towns 'disappear' along her routes. This came to a debate on whether she eats people or not. However it was made official that she doesn't eat people, it's just that the people shut off the lights and hide, so to prevent being seen.
Despite her scary appearance, she is actually kind to humans.