I've been looking at this too, as I'm about to be in a cabin in the woods for three weeks with only one other human being within 50 miles of me...
My initial thought is that I might want some NPCs to become a bit less fragile than others, but not at the MCs (sole) discretion. (After all geeks gotta be in the crosshairs.) So, I thought that I might make it based on actions of the PCs, in reward or consequence. Something like...
When you induce major fuckery (in the good sense or the bad) in an NPCs life, and that NPC survives and knows it was you, they get Hx with you.*
When a character with Hx of 1 to 3 is helping you, they give you +1 forward. If their Hx is 4+ they give you +1 ongoing.
When a character with Hx of 1 to 3 is interfering with you, they give you -1 forward. If their Hx is 4+ they give you -1 ongoing.
Characters with Hx take damage more like PCs than NPCs. They’re a bit tougher to kill off. Still in the crosshairs, mind you, but they take a little more harm before their head pops like an extra in Scanners.
NPCs don’t have negative Hx, nor does their Hx reset or anything like that. It goes from 0 to 4, and that’s that.
In the solo game, start off like 3 NPCs (who look like PCs by type but don’t have moves cause they don’t roll dice) with Hx 1 to the PC. The PC can then mess with their Hx as normal.
The Player should probably have first crack at introducing these NPCs, followed by the MC.
*Things that count as major fuckery:
- Fucking them
- Trying to kill them and failing
- Mind fucking them
- Healing them from some painful screaming death injuries
- Killing someone they care about while they're standing there watching
- Healing/saving someone they were good and serious about killing so that that fucker can come back and kill them now, thanks asshole.
- Making an example of them
- Anything that makes everyone watching wince and/or say "Jesus Cripes"
- Doing what Marie would do