stand-in, substitute, relief, reliever, backup, backup man, fill-in (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"

reliever, allayer, comforter (noun)

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

"a reliever of anxiety"; "an allayer of fears"

reliever, relief pitcher, fireman (noun)

a pitcher who does not start the game

reliever (Noun)

A relief pitcher.

reliever (Noun)

Someone who fills in for another.