Okay, so a demon OC backstory can be such a fun creative playground, but I see so many folks defaulting to like 'tragic past made them evil' or 'forgotten prince of hell' tropes. They're classics for a reason, but I think we can get weirder. The most memorable ones I've read tend to ask a really off-kilter question first. What if the demon wasn't fallen or made, but like, an emergent property? Like urban decay given a voice, or a collective of broken oaths from a single city neighborhood manifesting? That kind of thing immediately gives you unique rules and motivations. Instead of 'wants to rule the world,' maybe it just wants its specific block to stay beautifully, authentically crumbling, and sees gentrification as a holy war it has to fight.
Another angle that hooked me recently was reading the demon as a bureaucratic entity. Picture this: a minor functionary in the infernal civil service, whose entire backstory is about clawing its way up from the mailroom of the Ninth Circle through centuries of paperwork and office politics. Its 'demonic powers' might be super niche, like perfectly forging any signature or causing printer jams at will. The conflict comes when it gets assigned to corrupt a pure soul on Earth, and it's just... terrible at its job, or discovers it actually kind of likes the mortal world's inefficiencies. That sort of mundane-to-magical contrast builds immediate sympathy and humor, which makes the darker moments hit harder later.
The texture comes from the small, weird details rooted in that core concept, not the big tragic events. If your demon is a coalescence of abandoned promises, what does it physically collect? Maybe it hoards unused wedding rings or the dried ink of unsigned contracts. How does it perceive time? Maybe it sees the future moment a promise will be broken as a glowing crack in reality. Those specific, sensory details do more heavy lifting than any amount of 'and then hell tortured them for 300 years' ever could. It makes the backstory feel lived-in and the demon feel like a real, strange being with its own logic, not just a plot device with horns.