Who is Ruth's Possible from Never Let Me Go and what is their importance?

Ruth's possible is the unnamed woman, who is first seen by Rodney and Chrissie. Chrissie thinks she looks enough like Ruth to have been the original donor of her genetic material. The trip to Norfolk is undertaken to show the woman to Ruth. As the five clones get closer to the woman, they all become aware that she is not likely to be Ruth's possible at all.