I'd like to say something like "the complainer" for a one-off incident where a person complained about another person, but without implying that they complain habitually (i.e. complainer), and without implying there were legal proceedings (i.e. plaintiff, complainant).
plaintiff - the person in a legal proceeding who makes a charge of wrongdoing against another
complaintant - the party who makes the complaint in a legal action or proceeding
complainer - a person who makes frequent complaints usually about little things
Example sentence: The ____ accused Mrs. Jones of taking her keyboard. The ____ then proceeded to raise her voice at Mrs. Jones.