Piera basically joins the Royal Ribbon before the beggining of this story, and her master is the Dark Ribbon, pictured here in the bottom left:
Her orders are to become strong enough to take over the world. Piera goes along with this because she thinks it will ease the burden on her parents if their are no villains on the planet her kingdom is in. She was eager to join the Royal Ribbon when they gave her the offer because she was neglected by her parents due to them being busy saving planets all the time. At this point in the story, her father is the one that is home for once, but only temporarily. He is there to oversee her choosing a knight so that both he and the queen can have peace of mind in the Princess's safety. They leave her there to take care of the duties of that kingdom which is a responsibility they dump on all their offspring.
With that kind of pressure, she becomes easily misguided by the dark ribbon taking advantage of her good will and well-meaning disposition and turning it towards goals that would be more along the lines of a villain.
The only difference is that she is doing it for her parents while a villain would just be doing it for themeselves.
Anyways, this plot line would last the equivalent of maybe 3-5 volumes. It ends with the Dark Ribbon confronting Piera about neglecting her goals after seeing that her goals are starting to aim less and less at conquering the planet and more at helping it.
I've already drawn out how this plot line ends last year which is hinted at on page 8.
Basically, the Dark Ribbon forces Piera to fight Gofero due to him having goals opposite and effecting Piera's thinking. It's around that point that Gofero finds out/realizes that some of the villains in past fights where telling him that she was a "bad guy" due to her being part of the Royal Ribbon. {It's suppose to be a running joke, since Gofero thinks they are being metaphorical or some other misunderstanding. lol}
So after that fight, they defeat the Dark Ribbon and Piera is able to honestly be a good guy. They get the attention of the other members of Royal Ribbon for being strong enough to defeat the Dark Ribbon. So they became enemies they have to fight later.{The Queen of Royal Ribbon is in the background above the others, sitting on a throne. She's 10x more powerful than her strongest underlings. So she is a "late game" boss for lack of a better word.} Each of the "ribbons" pictured have their own small group of "lost princesses".
Anyways, the running joke about Gofero's obliviousness starts with the mini-villain that appears a few pages from now.
Anyways, what do you think of Piera's sub-plot? Would you rather she just be a real heroine from the get-go rather than a "heroine" who just happens to be defeating villains because their goals interfere with the plans of her "masters"?
It's a sub-plot since the real plot begins with the "Dragonhood". That's when the dung pie hits the proverbial fan.