From Halopedia, the Halo wiki
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Non-playable character (NPC), or Bots refers to any character in a gameplay scenario whose actions are not controlled by the player, but rather through an artificial intelligence. Several modes include NPCs including Firefight, Multiplayer and most prominently Campaign.
There are three types of NPCs: hostile, allied, and neutral. Hostile NPCs will attack the player on sight with whatever is available at their disposal, or deter the player in some way. Allied NPCs will actively fight the enemies of the player and generally help them. Neutral NPCs will not engage in any combat whatsoever.
NPCs on either side have the ability to enter vehicles on the map. This can give either side huge advantages. NPCs can enter the turrets and side seats of larger vehicles, and drive certain vehicles. Which vehicles and the NPC's skill with them, is dependent on which game is played.
NPCs have unlimited ammo for their weapons. However, they must reload once they are done their magazines. In Halo 2, Halo 3, Halo 3: ODST, Halo: Reach, and Halo 4 the player can switch weapons with allied NPCs, helping to resolve issues of low ammo or help optimize combat efficiency for your team of NPC. Conversely, you can obtain slightly more powerful weapons that may not be available on the ground.
Some may feature unique triggers and dialogue that differentiate them from "normal" allies because they are vital to the development of the game's storyline.
Every Halo game allows the player to be stealthy and eliminate enemies from behind without alerting other NPCs. This mechanic is extended to have special animations in Halo: Reach, Halo 4, and Halo 5: Guardians called assassinations.
In Halo: Combat Evolved, there are several types of NPCs. Some of whom are important story characters like Captain Jacob Keyes and 343 Guilty Spark. Keyes is only seen a few times, but if he is killed, the mission is failed and you must restart the level. 343 Guilty Spark is invincible and immune to any attacks on him during the game. Avery Johnson and Marcus Stacker are two others but they can be killed with no repercussions.
In Halo 2, all NPCs are invincible. They usually fight alongside the player and lead allies into battle. Most of the time they are also armed with stronger weapons and feature unique dialogue that is integral to the storyline.
In Halo 3, they share the same characteristics of Halo 2's NPCs, but when they take too much damage, they will fall to the ground, lifeless, and stay there until the battle is over. Soon after, they will stand back up and utilize the nearest available weapon, which usually tends to be their own.
In Halo 3: ODST, the same rules still apply from Halo 2, though certain NPCs are playable in some campaign levels (for example, Tayari Plaza is told from Gunnery Sergeant Edward Buck's point of view, but he is an NPC ally in NMPD HQ). The only NPC excluded from the "invulnerable" rule is Vergil.
In Halo: Reach NPCs function more akin to those in Halo 2; they are invincible however still react to incoming fire. Unlike in Halo 2, however, the player cannot swap weapons with allied those in NOBLE Team. Halo: Reach also features the Fireteam system, which is a unique way of recruiting friendly AIs.
In Halo 5: Guardians, both Blue Team and Fireteam Osiris are unique NPCs that are dubbed "Musketeers" by 343 Industries. The Musketeers can take damage and go into a "final stand" or "down-but-not-out" state from which they can be revived. Similarly, the Musketeers can revive players in the same state. Musketeers can also be ordered by the player to take vehicles, attack enemies, look at different objects in the environment, take weapons and move to different locations. In the gametype Warzone, players can fight a variety of different NPCs to earn points including "Legendary" enemies that are specific to the maps they are fought on.
List of Halo NPCs
Halo: Combat Evolved
Halo 3: ODST
Halo 5: Guardians