reliever
noun.
someone who takes the place of another (as when things get dangerous or difficult)
the star had a stand-in for dangerous scenes|we need extra employees for summer fill-ins
noun.
a person who reduces the intensity (e.g., of fears) and calms and pacifies
a reliever of anxiety|an allayer of fears
Synonym: allayer, comforter
noun.
a pitcher who does not start the game