Unless they are magically created.
Or have your soul linked to them... soul gems are quite common.
I think a GREAT idea would be to have a health bar (100% for everyone) and a second stamina bar or just use the stamina bar that exists. When the stamina bar goes down below 0 you get knocked out and npcs will leave you alone and players/npcs can loot you. After a few seconds you would regain consciousness but would be in a weakened state (so that you aren't invincible)
If players choose to attack you while you are unconscious or with "fatal wounds" it would dmg your health (the 100% bar) and if it went to 0% you would then die. Players who actually murder other players would have their soul tainted and they'd be flagged for life and suffer consequences -- you could use in game lore for this. This penalty would make hardcoe pkers think twice but it would still make it realistic in the sense that there are murderers in the world. mugging other players and taking their gear wouldn't cost you to lose your life like in other games, wouldn't be too unrealistic (no need for respawn) and could encourage rivalry (and some inevitable trolling, I know =( ... ) but it would give players an option beside killing everything lives always seem so meaningless in games where you just kill everything.
I know this idea seems complex but morrowind did it (sort of) so why not?