In a civil case, the complainant is the party alleging the offense and brining the lawsuit.
Merriam-Webster Online Dictionary
complainant (noun)1.
the party who makes the in a legal action or proceeding - complaint
2.
one who - complains
Merriam-Webster Online Thesaurus
complainant (noun)the person in a legal proceeding who makes a charge of wrongdoing against another
SYNONYMS:
plaintiff, suerRELATED WORDS:
accuser; litigant, party, suitor; appellant, petitioner, pleaderNEAR ANTONYMS:
accused