Nouns

(n)stand-in, substitute, relief, reliever, backup, backup man, fill-insomeone 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”
(n)reliever, allayer, comfortera person who reduces the intensity (e.g., of fears) and calms and pacifies“a reliever of anxiety”, “an allayer of fears”
(n)reliever, relief pitcher, firemana pitcher who does not start the game