La Pucelle takes place in a kingdom named Paprica. The story centers on a small church called the Church of the Holy Maiden, in the city of Pot a Feu. In addition to the normal roles and duties of a Church, they also have a group of trained Demon Hunters called La Pucelle. Two of La Pucelle's newest members are Prier, a sixteen-year-old girl, and her twelve-year-old brother Culotte.
It is said that the Dark Prince, favored servant of the fallen angel Calamity, will rise to scourge the land. But legend tells that when the Prince arises, a warrior known as the Maiden of Light will challenge him and restore the balance of Light and Dark. Prier hopes to be the Maiden of Light.
If you defeat certain powerful enemies in the Dark World/Netherworld (which requires a lot of Level Grinding), a band of demons will appear and declare Prier their new ruler... which Prier rejects out of annoyance after being teased by her teammates, and the game continues anyway, ignoring this interruption. However, Prier being a demon queen is Canon, as she puts in several appearances in the Disgaea series as "Demon Overlord Prier," a powerful optional character.
If she continues on past that, becoming even stronger, she triggers a Non Standard Game Over as the demons make her their new ruler and her friends abandon her. THAT is the canon ending, the one where she is half-demon with wings and a horn. Curiously, the plot still ignores this.