complainant

noun

1. One who makes complaint, one who complains.

   “1708-1714 Jeremy Collier, Ecclesiastical History of Great Britain Eager complainants […] in the dispute.”

   “Heading the complainants is Sir Edward le Breton, Lord of the Manor of Loders, and a member of Bridport Rural Council.”

   “The Southern Region takes, in the main, a candid line with its public. [...] An ill-informed attempt to blackguard the railway publicly is likely to see the complainant put politely—but very firmly—in his place.”

   “It's hard for Americans to understand the patriotism that can get bound up in place-names. We're a young country. We're also accustomed, in our cockeyed cowboy fashion, to everything else revolving around us, so we can afford to let slide the fact that, say, the Gulf of Mexico isn't called the Gulf of America. (Although, according to John Hébert, that is the pet issue of one frequent complainant to the Board on Geographic Names.)”

2. The party that brings a civil lawsuit against another; the plaintiff.
3. An alleged victim in a criminal investigation or trial.