Alright, then unless anyone objects, we'll set this in modern Japan. Here is a my proposal for a setting/premise breakdown (subject to change based on your input):
In the not too distant future, Asmodeus, the Prince of Lust, commanded his minions to break through the barrier between the human and demon worlds, intent on conquering the Earth and subjugating the human race. They were able to open four rifts into our world, suffering tremendous casualties in the process. But despite their losses, what remains of Asmodeus' army is more than capable of conquering the world through brute force. It is not merely that demons are more powerful than humans, though they are; as celestial beings, demons' bodies can't be harmed by mundane weapons, no matter how powerful.
There are two graces by which mankind might be saved. The first, is that Asmodeus wants to rule us, not to slaughter us. Rather than force an open conflict, and risk the deaths of millions or billions of his future subjects, he has set his forces on a campaign of corruption. It is slow and subtle, which gives those few humans who are aware of his machinations the time to fight back.
The second is Gemory--the Prince's second in command, who first pierced the barrier between worlds, at the cost of her life. Her body and armaments were recovered by humans, and her celestial essence was used to engineer weapons that could fight the demons. But, although every effort has been made to purify that essence, it still carries a trace of demonic corruption, that may yet prove to be the downfall of the heroes.
Again, open to suggestion or revision, but if you need setting info for your backstories, that's a good starting point. We'll be focusing on a rift in Japan; Asmodeus' forces are divided between the four, which is why you only have to deal with one general at a time. Whether your characters are already members of the resistance or if you want to be recruited when the RP starts, is up to each of you.
Also, freeko and Tenshi, I don't know how this will impact your timeline but high schools in Japan typically cover three years instead of four (10-12 grade), while 9th grade is the third year of middle school. But there are a lot of middle schools affiliated with particular high schools, with the assumption that graduates of the middle school will move onto that high school, so if you want your characters' relationship to stretch that far back you can certainly say the two affiliated schools share a campus.