If HBO is the God of Ti*s and Wine, then Ros is the figure they chose to come down and represent them on screen on Game of Thrones.
A character invented for the screen, there initially seemed to be little purpose to Ros other than helping the series fulfil its nudity quota (which was particularly high in the first couple of seasons).
Other than that, she largely served to assist in great exposition dumps, while they attempted to give her character a little more to do by having her working for Littlefinger and later spying for Varys.
There wasn't really much to Ros, who was there to be used in whatever capacity required in order to better serve other characters, and her scenes could quickly become irritating. The most memorable thing about her was her death, and we didn't even see that happen properly.