Halo Atonement is a first person shooter from the perspective of a United Nations Space Command Orbital Drop Shock Trooper, named Davian Miller. It takes place after the graphic novel Halo: The Third Battle of Earth, which explains more about the human civil war, and the eventual defeat of Major Wilson, and also, how The Covenant become involved in this game. The story unfolds over the course of seven extremely long missions.
- Captain Davian Miller - Davian is the main protagonist, player character and squad leader of the Special Forces Squad of the ODST 7th Platoon of the ODST 7th Battallion, replacing Major Williams after his death by Colonel Cody Thompson. He is a caring soldier, who watches over his squad with a careful eye.
- Master Sergeant Nathan Phoenix - Nathan is the second in command and communications (recon) officer of the team, him and Davian are known to be close friends.
- Corporal Jon Griffin - Jon is the squad sniper and medic. He is termed as being a 'wiseass' and has a bad habit of lying and disobeying superior officers.
- Gunnery Sergeant Ivy Raven - Ivy, Jon, and Davian have been friends since childhood. Ivy is the squad officer and lead 2nd rifleman, and is one of the more physically adept squad members.
- Sergeant Cody Wren - Cody is another one of the more physically adept squad members. Because of his keen eyesight, he is the squad spotter, which is why he is often teamed with Jon.
- A-Attack Command
- B-Fallback (Take Cover) Command
- X-Bring up Squad Commands
- Y-Switch to secondary weapon
- Y-Regroup Command
- RT-Fire Right Weapon
- RB-Reload Right Weapon
- LT-Fire Left Weapon / Throw Grenade
- LB-Reload Left Weapon / Switch Grenades
- LS-Crouch (Again to lay down)
- Prologue - Armor up, again.
- The Captain - Get used to some new shoes.
- Mombai Plains - Mount up, get to New Mombai.
- Keep it Clean - Cleanse New Mombai of Flood.
- New Mombai - Defend New Mombai from The Covenant.
- Ghosts of Tsavo - Cross the highway of Tsavo.
- City of Tears - Stop the train from reaching its destination.