RELIEVER
reliever, relief pitcher, fireman
(noun) a pitcher who does not start the game
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”
substitute, stand-in, 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”
Source: WordNet® 3.1
Etymology
Noun
reliever (plural relievers)
(chiefly, medicine) Something which relieves (pain, etc.).
(baseball) A relief pitcher.
Someone who fills in for another.
(slang, dated) A garment kept for being lent out.
Source: Wiktionary
Re*liev"er (-r), n.
Definition: One who, or that which, relieves.
Source: Webster’s Unabridged Dictionary 1913 Edition