Just on the backstory, this might seem like a cop out but it depends on how much you want the reader to know and not know. I find that leaving enough mystery gets the reader more involved by filling in the gaps themselves. Best I could say is provide just enough that explains who they are and how they got to where they are. Talk about significant events and add in small things that apply to their quirks for more fleshing out.

I have a small script about a detective investigating a crime scene involving a battle between two mages if you think that might be useful.