In said order... *clears throat*
1: It is called Soul Wavelength, and it is not ones life force, but rather the fountain of one's power. It can be considered part of your life, since if the soul ruptures, you die.
2: Depends on if your character is a demon, witch, or rogue meister/weapon. There is only two to three cases where a Weapon didn't require a Meister, and all of them are strong enough to be considered broken.
3: A demon's visibility is dependant on their situation. Chrona's black blood demon was easy to hide since he was a manifestation of her blood, but the Kishin clown stood out horribly because he was a product of madness.