Personally, the backstory I feel needed is to explain
- Why they don't seem so eager to return on their own world.
- Why the MC accept so easily the existence of succubi, magic, an other world full of fantasy.
I definitely don't see an ex military, police officer, and the like.
For the first point, I see someone lonely, with a boring job and no future project, no real ambition. So no strong attach to their previous life. It will explain why they have no urge to return home.
The start of the game feel like that. Well not totally for the lack of ambition, as they want to earn a promotion. But failed miserably, so their job isn't a strong motivation to come back.
And for the second, someone who was a fan of SF/fantasy (probably to escape their dull life). Which explain why they can easily accept all the magic/demonic stuff.
I think it's what is missing in the prelude of the game.