Things like characters are copyrighted to the works they appear in — in other words, I own, say, Jace Wayland. You are welcome to play around in my fictional world by writing fanfiction or creating other fan works, but you can’t use them in work you intend to publish, just like you can’t sell fanart or merchandise based on my work — or anyone’s work that isn’t yours. It’s just not a good idea, not for anyone. Publishers want to see you can come up with your own ideas and characters so you are much better off doing that!
“Full measures indeed of love and suffering…and bloodshed too: Fans will sail through and clamor for more.” — Kirkus