This "Reluctant Demon King (魔王 Maou) Heiress" trope seems to be a subversion of the classic "Hero vs Demons" fantasy genre. The demon king is gone, and the heir to the throne and/or the owner of Maou's power is a female, and she has almost no interest in doing evil things to humanity. Instead, she wants to mingle with humans, helps humanity prosper and is mostly in love with the shounen main hero.
There are several anime/manga and assorted media where the heroine is:
- Of a benevolent demeanor (not evil).
- Heiress of, or the demon king herself.
- Romantically involved with the MC.
- On the run from her duties as heiress/king (see #2)
- Strongly disinterested in pursuing the path of former demon kings.
As examples, we have:
Where did this trend start from?