I'm not trying to pee in your cereal but I also had this big, cobbled together, overplanned and impossible to make real ...thing when I was a teen. It was about demons and fighting and terrible stuff and I'm speaking out of sympathy here. Just asking a few questions to think about.
Structure your synopsis better. Don't mention "the nexus" without explaining what it is and if you do that you may not even have to name it. Too many secret organizations with cryptic names. Why is the hero an assasin specifically when the world is under attack from demons. Does he get hired by demons to off other demons as well? Why are their names in Japanese? "Kazuma" is just highlighting in really bright flourescent marker that you really liked ScryEd. Hanabi? What? Town? Event? What? If these are space demons from space make it clear so that anyone interested knows what sort of story you're going for. Urban fantasy, space opera or action horror. I'm not sure at the moment. Long serial, try writing a story arc first. Before that try writing a chapter/issue. Make that as a test and see how much effort and time it takes. Just saying that you should be prepared. A graphic novel of some 200 pages takes a year or two to make depending on speed. Since your artist is unpayed and not on a weekly schedule supported by assistants you may have to adjust your goals to get the story out in more thorough chunks.