Creating manga

That sounds like straight up slavery o.o and genocide by the humans. It's very black and white demons are inherently evil unless they are stupid pets, and humans for some reason need there slaves to even fight in the first place. There's nothing to look forward to in the plot. You focused to much on cool demon stuff, which is cool, but your theme goes nowhere. It's just mindless murder there's no emotion to it.

Now this guy knows what he's talking about it. I agree 100%. My piece of advice. Think not only of the story, but how your gonna tell it. A burger is good, but it's better with extras on top. Also, divide your stories in parts, as in into to conclusion stuff, and don't be cheap on details. Write up stuff even if it's not relevant to the story. That should help you get a better grip on what you're trying to tell.

Key is also right. I think the easiest story to do is the classic Gregorian. Put hero in normal world, have a call to action, a need to save something important to him, give him a goal, and make an interesting powerful enemy who is not inherently evil. Throw in some stuff about genetics or ninjas or demons or pirates and some other original developments and you have a good story. there's a few things you need to target in writing. 1. You characters need goals 2. Your scenarios need mystery 3. Your characters need passion 4. Everything needs layers of dimension. A.k.a Gray area. And what every writer needs. 1. Don't give up 2. Practice makes perfect 3. Question everything. Especially yourself 4. Don't be afraid to be weird. keep working buddy. your plots might just get lost in weak diction and syntax. I'll admit your spelling makes comprehension difficult for me, but I assume you have good reason for the bad spelling.