The Demon Country was the home of the spirit of Mouryou, a spirit which grew out of the evil of man and the dark side of the country's shrine maiden. The shrine maiden had the duty to seal away Mouryou and keep his evil contained. The country was allied with the Fire Country, hiring its ninja for protection.
The current leader of the Demon Country is unknown. However the current Shrine Maiden was charged with protecting the country so she may maintain the official position of authority.