From Halopedia, the Halo wiki
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Non-playable character, or NPC, refers to a character in a video game whose actions cannot be controlled by the player. However, these characters are often an important part of the storyline.
NPCs are only seen in gameplay with the player, and are usually either allied or neutral to the player. Classically, they feature unique triggers and dialogue that differentiate them from "normal" allies because they are vital to the development of the game's storyline.
In Halo: Combat Evolved, there are only two NPCs, 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. 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 NPCs, specifically those in NOBLE Team.
List of Halo NPCs
Halo: Combat Evolved
Halo 3: ODST
Halo 5: Guardians