The person who originally files a lawsuit or action, or appeals a lower court judgment.
Merriam-Webster Online Thesaurus
petitioner (noun)one who asks earnestly for a favor or gift
SYNONYMS:
petitioner, pleader, solicitor, suitor, suppliantRELATED WORDS:
beggar, mendicant, panhandler; cadger, moocher, schnorrer; asker, requester, suer