The most important figure in the Populist Party was a man named Tom Watson. He was a lawyer from Georgia whose parents had been rich slaveholders who lost everything because of the Civil War. So if you are asking about an individual, he is the one most commonly associated with that party.
If you are asking about who supported the party more broadly, the answer is farmers. The Populist Party drew almost all of its support from farmers in the South and in the West. In the 1892 election, for example, the Populists won Colorado, Kansas, Nevada and Idaho.
Populist party also called the Peoples party was formed in 1891 by joint initiatives of different political groups such as farmers alliances, labour groups and reform groups. It is difficult to name just one person or even small group of persons as organizer of populist party.
Some of the prominent leaders of the populist movement and the populist party include James B Weaver, William Peffer and Ignatius Donnelly. James B Weaver was the party's presidential candidate in 1892 elections and Wiliam Peffer was the party's first US senator. Ignatius Donnelly wrote part of the populist platform on 1892.