The human array of forces, sent forth to do battle with their Covenant counterparts. Know these well; they're the only things you're ever likely to see that aren't shooting at you.

The ranks of the bloodthirsty Covenant army. Rather than a single race of aliens, the Covenant are a conglomerate of creatures united in the cause of destroying the unholy. Consequently, each variety of their forces have various strengths, weaknesses, and characteristics; this is not an unchanging mass, but rather a dynamic, multifaceted army.

Not much is known about these parasitic lifeforms, other than the fact that they use the biomass of other living organisms as food. When a host body can be put to use, the Flood spores will possess the organism and enslave it through a method of mind control. The Flood do not appear until later in the game, when they are accidentally released by the Covenant.

The Forerunner, the beings that created Halo, are shrouded in mystery and confusion. These characters have connections with the Forerunner, and are cloaked in a certain sense of mystery.