is stan the main character in south park
Yes, he is.
Stan Marsh
 Marsh is a main character of the animated television series "South Park". He is voiced by and loosely based on series co-creator Trey Parker. 
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Stan Marsh [SEP] Introduction
Stanley "Stan" Marsh is a main character of the animated television series "South Park". He is voiced by and loosely based on series co-creator Trey Parker. Stan is one of the show's four central characters, along with Kyle Broflovski, Eric Cartman, and Kenny McCormick. He debuted on television when "South Park" first aired on August 13, 1997, after having first appeared in "The Spirit of Christmas" shorts created by Parker and long-time collaborator Matt Stone in 1992 ("Jesus vs. Frosty") and 1995 ("Jesus vs. Santa"). Stan is an elementary school student who commonly has extraordinary experiences not typical of conventional small-town life in his fictional hometown of South Park, Colorado.