r-o-o-k-i-e

When Spike rematerializes in the office of Wolfram and Hart after bringing down the Hellmouth in Sunnydale, someone else shows up too. A man no one recognizes, and who Spike can''''t seem to shake. A man who also appears to be a little insane too. Who is he really? Why is he following Spike? And why can''''t Spike get rid of him?