Err you positive - i swear i wasnt anywhere near my companion and she ded - i was too busy swimming around in some ruins and didnt notice there were any enemies - had to reload as i didnt want her to die as she was my characters wife.
I'm 100% possitive. I specifically let my NPC fight alone and watch what happens. Because I'm playing some sort of battle mage, my fireballs are the only thing I've seen permanetly put the NPC down.
I even watched my NPC fight a dragon and fall in battle.
If you didn't kill them, they will come back.
I do however think there is an issue when dealing with hoards of enemies. When the NPC hits ~10% health, they yield and stop fighting, cowering on the ground. Enemies immediatly stop focusing the NPC and begin focusing you. The NPC remains behind with low health. However, if the NPC is tanking several enemies, and all the enemies take a swing at the same time, she can be killed, as the enemies didn't have enough time to stop fighting the NPC.
Either way, the game saves a lot, and can be corrected if you lose the NPC.