This is a mood for sure! I like some aspects of this design - the palette is strong, and good weight on the lines. I'm hoping you can find a way to make them "legally distinct" from your average neighborhood ninja though - maybe push the forms a bit further or find some unique shape language for the character. Right now this reads more "foot soldier" than "main event". I'd also make it less busy - three swords doesn't have as much impact as, say, one gigantic sword, or one really cool sword with something unique about it. Finally I'd consider incorporating something more than just the flag's colors - Japanese culture is pretty famous, so work some aspect of that into it. Like, maybe make a list of "Japanese things" and try to add one or two of those elements into it, for example "sushi + ninja" or "giant death robots + ninja" or something. You have a very clean start here that looks good but push it a bit further to really make it explode!
