WordNet sense 1 (someone who takes the place of another (as when things get dangerous or difficult)):

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

WordNet sense 2 (a person who reduces the intensity (e.g., of fears) and calms and pacifies):

comforter, allayer

WordNet sense 3 (a pitcher who does not start the game):

fireman, relief pitcher