As an option, I do have a system-agnostic tool we could use that I've been wanting to try out. It may help us to flesh things out a bit between your characters, and it won't omit or replace anything you've already decided about your background and relationships.
It's called
Decuma; it draws cards from a Tarot deck and presents questions that you'll answer about your characters and details about the setting.
It may take us a few posts to get through it since it involves each of us drawing cards and collaboratively answering the questions they pose, but if you're interested in trying it out, I'd love to use it.
However, if you'd prefer just to get right into the game and figure details out as we go, that would be fine, as well.