Who is Count Rugen/Six Fingered Man from The Princess Bride and what is their importance?

Count Rugen is the only Count in Florin. The title was given to him by Prince Humperdinck since Rugen is the Prince's only friend. The Count is central to the plot for two different reasons. Rugen is the man who discovers Buttercup's beauty and tells the Prince about her. Rugen is also known as the Six Fingered Man. He has six fingers on his right hand.