complainer vs bellyacher

complainer and bellyacher both are nouns.

Word	Noun	Adjective	Verb	Adverb
complainer	Yes	No	No	No
bellyacher	Yes	No	No	No

As nouns, complainer and bellyacher are synonyms defined as:

complainer and bellyacher: a person given to excessive complaints and crying and whining

Other synonyms of complainer include crybaby, grumbler, moaner, sniveller, squawker, whiner.

complainer (noun)	bellyacher (noun)
a person given to excessive complaints and crying and whining	a person given to excessive complaints and crying and whining