 Halo 3 's protagonist is Master Chief Petty Officer John @-@ 117 , a surgically enhanced supersoldier known as a " Spartan " . The Chief fights alongside the Arbiter , a disgraced Covenant Elite commander . Two other Elite characters , N 'tho ' Sraom and Usze ' Taham , appear as the third and fourth players in cooperative play . Supporting characters from previous games return , including human soldiers Avery Johnson and Miranda Keyes . The Forerunner artificial intelligence 343 Guilty Spark , who tries and fails to stop the Master Chief from destroying its ringworld in Halo : Combat Evolved , also makes an appearance . Also playing a role in the story is the Flood entity known as the " Gravemind " . In Halo 2 , this leader of the Flood escapes from confinement on Installation 05 , invades the Covenant mobile capital city of High Charity , and captures Cortana , a human @-@ created artificial intelligence . 
