reliever

The noun has 3 meanings:

Meaning #1: someone who takes the place of another (as when things get dangerous or difficult)
  Synonyms: stand-in, substitute, relief, backup, backup man, fill-in

Meaning #2: a person who reduces the intensity (e.g., of fears) and calms and pacifies
  Synonyms: allayer, comforter

Meaning #3: a pitcher who does not start the game
  Synonyms: relief pitcher, fireman