Ashita no Kyouko-san

The ruler of the demon world has come, with only his trusty assistant, to conquer the modern day human world! It's a little rough from the start, because most people just seem to think he's a cosplayer. Also, due to the worst recession the demon world has had in 500 years, though, he's a little underfunded. For his conquest, he's got to fight the Hero of the human world. She's Ichikawa Kyouko, a distractingly pretty, tough, modern-day girl who happened to pick up the giant sword that marks her as the chosen one.
(Source: MangaHelpers)