Non-playable character

Non-Playable Character, or NPC refers to a character in a video game whose actions cannot be controlled by the player and is a vital 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 the Halo 2 NPC's, 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 any nearest weapon, more likely their own.

List of Halo NPCs
Halo: Combat Evolved
 * Captain Jacob Keyes
 * 343 Guilty Spark
 * Marines

Halo 2
 * Commander Miranda Keyes
 * Sergeant Major Avery J. Johnson
 * SpecOps Commander Rtas 'Vadumee
 * 343 Guilty Spark
 * Brute Chieftain Tartarus (Tartarus, other than the Heretic Leader, that is the only NPC that can be killed.)
 * Heretic Commander Sesa 'Refumee (only considered one because of his plot point, and the fact he is invincible until the battle with him.)
 * Marines

Halo 3
 * The Arbiter (save for Co-op)
 * 343 Guilty Spark (immune until the final battle)
 * Sergeant Major Avery J. Johnson (immune until Guilty Spark kills him)
 * Commander Miranda Keyes (immune until Truth kills her)
 * Marines