The simple answer is that it’s totally up to you. Because in the long run you’ll revise both so much it won’t matter what you did first.

I have created characters with no back story, threw them in a scene, watched how they reacted, then created a back story for them. Then during a rewrite of that scene, they acted a little differently because of their back story. And they way they acted gave me some ideas for adding depth and texture to their back story. And so on.

And the exact same thing happened when I created a back story for a character before throwing them into a scene.