reliever vs hurler

reliever and hurler both are nouns.

Word	Noun	Adjective	Verb	Adverb
reliever	Yes	No	No	No
hurler	Yes	No	No	No

As nouns, hurler is a hypernym of reliever; that is, hurler is a word with a broader meaning than reliever:

Other hypernyms of reliever include pitcher, twirler.

reliever (noun)	hurler (noun)
a pitcher who does not start the game	(baseball) the person who does the pitching
someone who takes the place of another (as when things get dangerous or difficult)
a person who reduces the intensity (e.g., of fears) and calms and pacifies