She's a bit like Elmo, in the sense that she gradually gained more prominence and popularity over the course of more than a decade. She's developed a lot over the years too, and I think she's one of the more well rounded of the show's main characters.
Carmen Osbahr doesn't get enough credit for how charming she makes Rosita. You can tell from her earliest appearances, she really stood out from the many short lived characters of the 90s.
As far as Murray goes, a lot of people foresaw him as becoming the new Elmo, so-to-speak, what with him becoming the sort of unofficial host of the show, getting his own segment, getting wrap-arounds and framing devices. Heh, I still remember when my friend Kevin Magpoc did this strip in his webcomic years ago: