is the whole story is about a period? I don't get it, a cute grill is a demon? really don't get it
Basically, I think you get a demon trait passed down from your parents, refer to biology if you want to know how trait inheritance works, and you go through the Usa thing or whatever it was at random moments (or maybe it was a set time, I didn't really like reading the back story) so you need a scabbard, a person who needs to calm you down or kill you if things get really bad. The demon is the sword and the friend (or girlfriend) is the scabbard, if you wanted the analogy in the story. This particular girl just goes through the Usa thing easier when she's on her period. Oh yea, I really like your profile picture by the way.
I really wish this had a continuation, it would've been an interesting manga