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Essential characters refers to non-player characters, which cannot be killed either by the player or enemies. Starting in Oblivion, their Health may be damaged, but it will never reach zero. Characters are generally only flagged as essential if they play major roles in quests. Distinct personages, such as Countesses and Jarls, are flagged as essential regardless of their quest involvement. Characters may be "unflagged" after certain events occurs, meaning they were once invincible and no longer are.