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, suppliant

RELATED WORDS:

beggar, mendicant, panhandler; cadger, moocher, schnorrer; asker, requester, suer

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Petitioner