noun (n)

• someone who takes the place of another (as when things get dangerous or difficult)(noun.person)

  Synonym:
  backup, backup man, fill-in, relief, stand-in, substitute
  

  source: wordnet30
• a person who reduces the intensity (e.g., of fears) and calms and pacifies(noun.person)

  Example:
  A reliever of anxiety.
  

  Synonym:
  allayer, comforter
  

  source: wordnet30
• a pitcher who does not start the game(noun.person)

  Synonym:
  fireman, relief pitcher
  

  source: wordnet30
• One who, or that which, relieves.(noun)

  source: webster1913