Non-playable character

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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 three NPCs, Captain Jacob Keyes, Avery Junior Johnson 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. And Johnson 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 the 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 any nearest weapon, more likely 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 Buck's point of view, but he is an NPC ally in NMPD HQ). The only NPC excluded from the "invulnerable" rule is Vergil.

Halo: Combat Evolved

 * Captain Jacob Keyes
 * 343 Guilty Spark
 * Staff Sergeant Avery J. Johnson

Halo 2

 * Master Gunnery Sergeant
 * Commander Miranda Keyes
 * Sergeant Major Avery J. Johnson
 * Spec Ops Commander Rtas 'Vadumee
 * 343 Guilty Spark
 * Brute Chieftain Tartarus
 * Heretic Commander Sesa 'Refumee (Invincible until cutscene)

Halo 3

 * The Arbiter (can still be played in mulitplayer mode)
 * 343 Guilty Spark (Invulnerable until cutscene)
 * Sergeant Major Avery J. Johnson (Invulnerable until cutscene)
 * Commander Miranda Keyes (Invulnerable until cutscene)

Halo 3: ODST

 * Gunnery Sergeant Edward Buck (playable in Tayari Plaza and Kikowani Station)
 * Taylor H. "Dutch" Miles (playable in Uplift Reserve, and ONI Alpha Site)
 * Michael "Mickey" Crespo (playable in Kizingo Boulevard and Firefight)
 * Kojo "Romeo" Agu (playable in NMPD HQ)
 * Captain Veronica Dare
 * Unnamed NMPD Officer (Invulnerable until descended to Sublevel 09 which results with a scripted death or upon discovering Dr. Endesha's body if the player has collected 29 audio logs)
 * Vergil