Essential

Quest essentials are items and characters in Fallout 3, such as Button's Ink or James. that are treated differently to prevent the player character from rendering quests permanently incomplete, such as the Quest items cannot be dropped, sold or stored in containers. Essential characters cannot be killed, they will be rendered temporarily unconscious and later awake with full health. Contrary to ingame information these items do not actually weigh anything.

PC owners can manually set characters essential via the console by typing in "setessential 1"