Which character becomes a recurring main boy after Kenny's season 5 death?

Butters Stotch becomes a recurring main boy after Kenny’s season 5 death. As Kenny’s role temporarily receded, Butters moved from a supporting classmate into the central group of boys alongside Stan, Kyle, and Cartman.

Kenny’s apparent death at the end of season 5 created an opening in the show’s regular boy ensemble. Butters had already appeared repeatedly, and his innocent personality, strict home life, and vulnerability to Cartman’s schemes made him a natural replacement for many of Kenny’s story functions. His increased prominence began especially in season 6.

The change did not mean Kenny disappeared permanently: South Park later restored Kenny to the main cast. Butters nevertheless remained a major recurring character and eventually received many storylines of his own. Token, Clyde, and Craig each became important recurring students, but none took over Kenny’s place in the central boys’ group in the same way.