Julian Sark was an elusive criminal affiliated with SD-6, The Covenant, and Prophet 5. He was also an avid follower of the work of Milo Rambaldi. He was notorious for changing alliances, offering his services to anyone who paid enough money (or agreed to let him stay alive) - even the US government. As stated by Anna Espinosa, Sark was a man so ruthless in his shifting allegiances that he killed his own father. He was skilled in evading capture, managing to stay active years after the destruction of APO despite two previous CIA imprisonments.