May have been mentioned, but an XXL waterproof poncho can be a life saver. Many uses but the big one is to avoid going into shock if you get hurt and/or hypothermic. Many uses for a good waterproof poncho, they are often inexpensive, light and easy to pack. Think of them as a small one man tent, preserves body heat. breaks the heat stealing wind, can easily be made into a litter etc.

A single stick as a tent post, any handy overhead cover (Pine Tree) and a single candle will keep you warm for hours.

I'm a cold-weather person, I've spent weeks outdoors in the snow. It only takes once to make you a believer about hypothermia. Under the right circumstances, anybody is vulnerable. I learned that lesson the hard way, almost died.