Definition of "complainant"

Eager complainants […] in the dispute.

1708-1714 Jeremy Collier, Ecclesiastical History of Great Britain

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.

1962 August, “Let's have plain speaking”, in Modern Railways, page 73