You mean Carrot?
Well after Guards! Guards! where Terry used him as a means to show the reader what Ankh-Morpork looks like from an outsider's point of view, Terry then doesn't seem to know what to do with him.
So he changes the character and gives him a love interest and we all know that Angua is far more interesting than Carrot.
His character just doesn't work because Terry has set too many limits on him. I also don't like the fact that we are constantly told that everyone loves Carrot. Why? Just because I'm told that doesn't mean that I have to like him too.
It's clear from the later books that Terry still doesn't know what to do with him, so he's stuck him in the background. And Vimes and Angua are the main people we see the Watch through.
But I could go on knocking Carrot all day long.
"Goodness is about what you do. Not what you pray to."