| Master Chief | Spartan 117- main character of the Halo series |
| Captain Keyes | Captain of the Pillar of Autumm |
| The Covenant | Highly advanced alien race built around belief of the "Great Journey" |
| The Arbiter | Fallen Elite Commander accused of heracy. Later joins the Humans |
| Elites | Race of the Covenant. The Covenant's most powerful military force. Highly trusted as Guardians of the Prophets. Join the Humans after the Prophets betray them |
| Brutes | Primitive race of the Covenant. Use barbaric weapons and replaced the Elites in Halo 3 |
| Cortana | Highly advanced AI that usually goes with Spartan 117 in his helmet |
| Miranda Keyes | Daughter of Captain Keyes. Led assualt to stop the second Halo |
| Sergeant Johnson | War- hardend veteran that miracously survives Halo 1. Is known as Sergeant or Sarge until Halo 3 where he is promoted to Sergeant Major. |
| UNSC | United Nations Space Command. |
| The Pillar of Autumm | Ship that crashes on the first Halo. Commanded by Captain Keyes |
| The Forerunners | Extremely advanced alien race that built the Halos and the Ark |
| The Moniter | Forerunner product known as 343 Guilty Spark and "the Oracle." Absorber of knowledge, this floating ball knows all the secrets of the Halos and acts only on protocol. Is the Librarian of Installation 04(the first Halo) |
| Flood | Parasitic zombie-like creatures that can transform any living thing into one of their ranks. The Halos were originally built to destroy all life thus starving these creatures |
| Halo | Ring-like superstructure built to end all sentient life |
| High Charity | Holy City and capitol of the Covenant |
| Prophets | Supreme Covenant leaders. Preachers of "the Great Journey" |
| Grunts | Small, insignificant units of the Covenant military. Can use a variety of weapons and vehicles but are extremly vulnerable and cannot function without strong leadership |
| Drones | Bug-like flying creatures rarely used in fighting. Fight in large numbers of around 30 to 50 |
| Jackals | Marksmans of the Covenant military. Seen with a shield in close combat or a sniper in long range combat |
| Hunters | Creatures that are built entirely of a sophisticated worm-system. Have heavy armor plating and carry fuel rod cannons |
| Gravemind | Intelegent central nervous system of the Flood. Has grown and taken over High Charity |
| In Amber Clad | Star Cruiser commanded by Miranda Keyes. Seen in Halo 2 |
| Scarabs | Huge four-legged war machines of the Covenant. |
| The Ark | Forerunner structure that can remotely activate all the Halos |
| The Prophet of Truth | Leader of all the other Prophets. |
| The Truth and Reconciliation | Covenant battle cruiser in Halo: Combat Evolved (The first game). Captain Keyes is captured and killed on this ship |
| The Library | Forerunner instillation on all of the Halos that houses the key to activating Halo. |
| The Index | The Key to activating the Halos |
| The Cartographer | The building that houses the map of the Halo installation |
| The Control Room | Room where the Halo installation is monitered and activated |
| Taurtarus | Brute Chieftan in Halo 2 |