The protagonist is a Maou, a demon king (or king of devils etc). He might've just escaped from being sealed for centuries and aiming to regain his former power (and enslave those who thwarted him the last time), just finally decided on taking over the world... or he might be a Maou who loves small animals and nature in general, hates mankind because they pollute it, and who ends up living together with a group of humans to see if they're really that bad. Do note that this tag is only intended for actual demon kings, and not for characters bearing Maou as a title or a nick. The character needs to have fantastic, demonic properties.