My Devil Is Such an Angel It Hurts

Story:

The Dark Knight Jade is tasked with guarding Princess Vignetta, whose mother was the slain Demon Lord, as this was her last wish before perishing at the hands of the Hero. The Princess is but a sweet small child brimming with virtues, so Jade wonders: When it comes to the time, will the Princess side with evil like her mother did and fight the Hero? A girl this sweet, no less?