Stevens suddenly started demanding a backstory all his own, and it sort of took off from there.

That's basically what's happened with Hillary. I watched "Tomb Raider" just wanting to see the actor's part, and the character moved into the squishy part of my brain with some thoughts about his own development.

I don't count Gillette & Groves as they were pretty fully developed fandom-canon characters before I came onto the scene.

In fandom, sure. But in canon, nothing much was done with them at all. So I still count them as minor characters you can play with like they're your own. :-)