Since we are talking R.S., it has to be what inspired the character.
My WiP is as mechanically constructed as you can get. I realized my character had a half-year where nothing happened between stories, and that got me thinking.
The difficulty here it that because it would be both a sequel and a prequel, I needed the story to have a reason never be reported while still being knock your socks off. Something that could still be shoehorned into later stories with a phrase like "Oops, that's a state secret. Don't repeat that," and not cause a disruption.
(I actually did. You know. Shoehorn that line into a later story, before completing this WiP.)
So what ideas did I throw at the wall?
An invasion of Earth nobody knew about (which comes from five chapters I never continued). My devil-girl had to had to stop it. I wanted to make it useful for ongoing stories by getting the devil-girl back on the main antagonist' radar by my devil-girl inadvertently doing the MA an enormous solid (i.e., saving Her ass).
Check. Check Check.
Didn't quite take off. My devil-girl wasn't biting. What was the story idea missing?