Titus Welliver played Man # 2, aka The Man in Black, aka Jacob’s Nemesis, aka “Esau,” in the Season 5 finale. In the last minute of this video from TV Guide (via latestlost), Titus talks about his character’s lack of a name. He also says that there is a greater power that he and Jacob answer to — which is something I had suspected earlier.
Photo from Lostpedia