My brother Fred and I were very close as we grew up in Perth. One abiding consideration was that our older siblings were much older than us so we were thrust together. We were roughly three years apart in age, but that translated to two years at school.
Characterisation is at the heart of any really good story as far as I’m concerned. Real people dealing with real situations. Or sometimes (as in speculative fiction) not-real situations. That’s when creating believable characters becomes absolutely crucial. If your characters aren’t believable, your reader won’t relate to them.
I’m busy at the moment doing some massive rewrites of early work and I’ve also spent some time doing some beta-reading. And backstory is the thing that sticks out.