Who was Sifo-Dyas? was closed as general reference. The general reference here seems to be the Star Wars Wiki page. Hmm, ok, but the asker evidently expected an answer relating to the movies, not the expanded universe, and it came as a bit of a surprise that he had to reach for non-movie material to get an answer. I don't see how that reference explains this clearly.
Related meta discussion: What sites should be considered general references?