early 15c., in law, "one who commences a legal action against another, one who makes a formal complaint in court," from Old French complaignant, present participle of complaindre (see complain). The present participle also was used as a noun in Middle French.
late 14c., compleinen, "lament, bewail, grieve," also "find fault, express dissatisfaction, criticize," also "make a formal accusation or charge to an authority," from stem of Old French complaindre "to lament" (12c.), from Vulgar Latin *complangere, originally "to beat the breast," from Latin com-, here perhaps an intensive prefix (see com-), + plangere "to strike, beat the breast" (from PIE root *plak- (2) "to strike").
The sense evolution is from "expression of suffering" to "grievance; blame." The transitive sense of "lament" died out 17c. Also from late 14c. as "utter expressions of grief or pain," hence, chiefly poetically "emit a mournful sound" (1690s). Related: Complained; complaining.
<a href="https://www.etymonline.com/word/complainer">Etymology of complainer by etymonline</a>
Harper, D. (n.d.). Etymology of complainer. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/complainer