Not only do your characters have a history, they also have a history with each other. It’s important to know not only how your characters relate to your MC and/or your antagonist, but everyone else in the world you’re building.
You’ll also want to consider how their relationships will change over the course of time—especially over the course of your novel. This is called a relationship arc, and it’s just as important as your plot arc and individual character arcs. And those arcs should all weave together in a fluid way.
Your characters impact each other. Their relationships impact your plot arc. Your plot impacts your characterization arcs.
And away we goooooooo!
That diagram I mentioned earlier this month? You’ll find an example of one below. Enjoy!