Petitioner

One who presents a formal, written application to a court, officer, or legislative body that requests action on a certain matter.

In March 1998, Aaron Wallace, president of the National Education Association, announces the collection of more than 400,000 signatures on a petition requesting increased spending on education in the state of Florida.
AP/WIDE WORLD PHOTOS

In legal proceedings initiated by a petition, the respondent is the person against whom relief is sought, or who opposes the petition. One who appeals from a judgment is a petitioner.