What stat determines...

[Roman Lodestar]

How much pain (or injuries) you can resist before blacking out? I have been testing some characters against robbers and they pass out after three or four minor injuries or one serious while NPC can resist many more hits (Njerpez are a good example of this). I find this unnerving since player character are fragile like a hare and can make combat more punishing.

[PALU]

I suspect it's endurance. I had a character with a low starting endurance and an endurance of about 50% after both game courses. Apart from a significantly slow fatigue recovery than normal (which improved after the courses), that character also fainted a lot, basically every time taking a wound (mostly friendly fire while fighting robbers), which also resulted in the final demise (a bite in the arm by a wolf, fainted, never woke up. The dog apparently did nothing). It can be noted that that character was as small as you can get them and with a middling strength, which might be factors.

My other characters have had good or very good endurance and have rarely fainted from pain (they all faint eventually from a combination of fatigue and abuse when robbed and resisting, of course).

The first rule when it comes to combat on life and death is that there is no such thing as poor sportsmanship. Back out if you're unsure about the outcome (and are able to), as living to fight another day beats a career as a carcass. Attack from hiding and from range. Use hit & run tactics. Gang up on the enemy...

[Roman Lodestar]

The characters I tested were owl-tribe, I suspect Kaumo are the master race of the Unreal World since I think weight, height and physique are also taken in account. I remember one human companion fainting by just a single shallow cut to the abdomen, if only robbers were like that I'd justify their sheer numbers.

Edit: Yep, definitely must be endurance after further testing, if only fleeing from combat was feasible...

[Acolyte]

The characters I tested were owl-tribe, I suspect Kaumo are the master race of the Unreal World since I think weight, height and physique are also taken in account. I remember one human companion fainting by just a single shallow cut to the abdomen, if only robbers were like that I'd justify their sheer numbers.

Edit: Yep, definitely must be endurance after further testing, if only fleeing from combat was feasible...

If it follows the TT rules then yes, it's endurance with a fair amount of RNG mixed in. When you have an endurance of, say, 12 and you're rolling 3d6 trying to stay under there's a fair chance you're going down. Your physical penalty also adds to the roll (in TT it's +1 per full 10 points) so if you're injured or fatigued you go down faster.

This is only if you get hit, though. Owl Tribe with a bow are brutal.
   - Shane

[PALU]

You shouldn't try to flee FROM combat, you should withdraw BEFORE combat, which makes it harder to determine what's the correct action. That's not always possible, of course, but you frequently have the option not to confront dangerous opponents.
The only combat you can flee from is basically the one in which you have the upper hand, but an exhausted enemy with a bow and a (virtual) quiver full of arrows is still dangerous, so if you're not equipped with a bow (and arrows) and have taken more than light damage you should at least consider whether to withdraw.

Edit:
Fortunately, hired NPCs seem to be as resilient as Njerps and robbers. I just suffered from vermin problem in my farm plots (a Njerp warrior equipped with bow and shield, plus scimitar), so I hired an adventurer to help removing it, as my character isn't a great warrior and is injured as well. The end result was an adventurer with 10 wounds, one of which was serious, without passing out, a dog with at least 3 bad wounds, and a PC with two additional wounds, plus a dead Njerp, at the end. I should have hired 2 adventurers, and would have, if they didn't keep refusing, and 2½ to 1 seemed like good odds.

Injury doesn't seem to affect fatigue buildup when swimming, and I haven't seen it for other tasks either (but swimming is particularly easy to spot at low skill levels). Injury definitely lowers movement speed and gives success penalties to heavier tasks (very easy and possibly easy seem to be unaffected by encumbrance and injury, as long as an essential body function isn't disabled).

[Pongo]

a 50% or 9 endurance is terrible.
Pretty easy to fail a shock check when you only have to beat a 9 on 3 die 6 and you are adding physical penalty of wounds and load to it.
Play with a 50% dexterity and you will be dropping your weapons on most hits to an arm. Or a 9 agility and be falling with most hits to your legs.
Play with a character with 15+ endurance and you will have some sustainability

[Acolyte]

a 50% or 9 endurance is terrible.
Pretty easy to fail a shock check when you only have to beat a 9 on 3 die 6 and you are adding physical penalty of wounds and load to it.
Play with a 50% dexterity and you will be dropping your weapons on most hits to an arm. Or a 9 agility and be falling with most hits to your legs.
Play with a character with 15+ endurance and you will have some sustainability

Very true! Keep in mind that - in TT at least - shock rolls (the ones that knock you out) range from 1d6 vs End (minor cut to the scalp as example) all the way to 5d6 vs End (grievous blunt to the eye for example). At 5d6, no-one's having a good time, but that's expected for such injuries.

Oh, and BTW Grievous blunt to the Groin is 6d6 vs End, but who'd want to remain conscious! 

   - Shane