I would argue that this could go in the literature board, unless you're writing for the manga and not for the story itself.
In which case, I don't have much to say to you.
A story should be illustrated - you shouldn't write a story with just the illustration in mind. If you're making up a plot just because you like manga, you're in danger of producing shallow characterization and storylines. You'd be restricting yourself to the standard you see in manga, which would end up more like replication than originality.
See
>>6 for exactly what I mean. The fact that the story is being described by a sweeping genre name and what romantic subplot is involved means it's 90% fanservice and 10% vested interest at best.
I'll say more in the lit board if you're actually interested in making a coherent story with reasonable characters. Otherwise, have at it, draw a long haired pretty boy with a dark past and their Manic Pixie Girl romantic interest fighting dark and edgy silver haired bishie demon bad guy man.