(a.) One who gripes; an oppressor; an extortioner.

accusant accuser allegator appellant bellyacher claimant complainer crab crank croaker delator faultfinder frondeur griper grouch grouser growler grumbler impeacher impugner indictor informer kicker kvetch libelant malcontent murmurer mutterer party petitioner plaintiff prosecutor querulous person reactionary reactionist rebel sorehead suitor the prosecution whiner