The essence is this.
After his sisters death at the hands of a demon, a young man searches for the truth. He finds it turns out she was a magic girl.
The story goes from his adventures in order to find more about the magic girls.
The ability he creates in order to steal the powers from both the good, and the dark magic girls. Till his eventual creation of a magic boys, and the war between both groups.
this is basely a massive crossover
If you want to find a co-writer, you're gonna need to put in a lot more work. I'll break it down.
The plot is too simple. It's barely even two lines. How can you expect anyone to be interested enough by it to want to contribute in a meaningful manner? You're not telling us anything. No information on the setting, no information on characters or goals, or motivations. And what you have told us is fairly cliche. Sister dies, brother wants to find the truth. We've seen it so many times before. What's your spin on it? Why does your version stand out from the others? The obvious answer would be ''magic'' but you've not got into depth with that. What kind of magic system are you running? Why are there magical girls but not magical boys? Point of fact, what even is the setting? If demons exist, are there spirits too? Are some good? Is there an eternal war? Is religion the truth? Can holy water repel demons? So many questions you haven't answered. You haven't actually told us anything. What's the plot? You've told us how it starts, but how does it progress? What are the personalities at work? Why is the brother looking for the truth? Is it revenge? Did he love his sister? Did he hate her, and is annoyed he wasn't the one to kill her off? We don't know.
Tell us more about the setting, tell us about magical girls and their interaction with the world. Tell us
about the characters, not just facts relating to them.
Why does he create magical boys? What's the purpose of it? Why is there a war, why are the two sides fighting at all? You've not named any other characters, or plot points at all.
As for the crossover aspect? Trust me, you don't wanna do that. I am not going to say that massive crossovers don't work (Though, I can still think it) but even if they do, they require so much skill, so much writing ability, so much effort dedicated to making sure that no aspect overshadows the others that it really isn't worth it.
Basically, you sound like you had an idea, and rushed here. What you should have done, was spend time fleshing it out in a major way, turning it from an idea into a full world, and then you could come here (Or rather, to the creative writing section) and be able to answer all the questions I asked. Maybe then you could find someone to help. But right now? People are gonna take one look and go ''If he can't even put any effort into this, why should I?''
