The main character of "Halo 3" is Master Chief. Master Chief is a super soldier, the last surviving member of the Spartan Project. The Spartan Project was started by humans to genetically, using genes, make soldiers who were stronger, faster and tougher than any normal human. Another character from Halo 1 and 2, controlled by the second player in a co-op game is "Arbiter", the leader of the alien Elites. In "Halo 2" he is an enemy but in "Halo 3" he joins forces with the humans. Other returning characters include the veteran human soldier Ser"geant Avery Johnson", the AI "Cortana", the robot "343 Guilty Spark" and "the Gravemind", the dangerous leader of the Flood.