A miko is a shrine maiden in the Shinto religion. Given the strength of Shinto in Japan and the position shrine maidens hold in it, it is no surprise that they are a common character type in anime. Straight portrayals and dramatizations are used just as frequently as a priest or nun would be in North American or European media. If a main character, she might be endowed with mystical powers in order to fight demons, hang Ofuda so that she won't have to fight the demons in the first place.