Answer:
I am not alive, though I never die, to no one I've ever told a lie, today I can be god's warrior, and serve him with no horror, but tomorrow I might be devil's knight, for I am what ends the light.
You use it between your head and your toes, the more it works the thinner it grows. What is it?
I have joy in bringing two together, but darning my existence! My life hangs by a thread, filled with ups, downs and resistance!
I start in little but I end in full, you'll find me in half and complete.
What is broken if you say its name?
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.