reliever

(noun) someone who takes the place of another (as when things get dangerous or difficult). Synonyms: stand-in, substitute, relief, backup, backup man, fill-in. Examples: "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. Synonyms: allayer, comforter. Examples: "A reliever of anxiety." "An allayer of fears."

(noun) a pitcher who does not start the game. Synonyms: relief pitcher, fireman.