How did jessie become a main character in toy story?

Quick Answer
Jessie became a main character in Toy Story after being introduced in Toy Story 2, where she was created to add a female character to the franchise, with development led by John Lasseter, Pete Docter, Ash Brannon, and Andrew Stanton.
Explanation
Jessie first appeared as a central character in Toy Story 2, created to address the need for a prominent female character in the Toy Story franchise. Her development was driven by Disney executives and Pixar talent, including John Lasseter, Pete Docter, Ash Brannon, and Andrew Stanton, who shaped her backstory as a cowgirl rag doll abandoned by her first owner. The character's introduction marked a significant evolution in the series, expanding its narrative scope and emotional depth. Following her debut, Jessie continued to play a leading role in subsequent films including Toy Story 3, Toy Story 4, and Toy Story 5, establishing herself as an integral part of the Toy Story universe.