Non-Playable Character

Non-Playable Character or NPC for short, is a generic term that refers to entities in MMORPG that are not controlled by Players.

Background
In YGGDRASIL, NPCs exist within the game to provide support to the players. Their purpose was to deliver quests, act as support characters, etc. NPCs were programmed with limited artificial intelligence to interact with players. Within the game, these artificial intelligence can only move accordingly to their program. In other words, they were the same as moving mannequins, only acting on pre-programmed action.

Strangely in the New World, NPCs have undergo a miraculous change. Now possessing free-will, they are able to freely walk beyond the limits of their original programming and engage with a player like Momonga and inhabitants of the New World. Interestingly as shown by the example of the NPCs of Nazarick, particularly the custom-made NPCs, they have gained the personalities of their creators, adopting their manners and beliefs.

Likewise to YGGDRASIL, NPCs that were transported to the New World are unable to go beyond their specialized level as it will stay the same by pre-program. However, NPCs like Narberal Gamma can still obtained enough battle experiences to learn from and adapt to instead despite not being able to level up.

Spawned NPCs
Spawned NPCs are entities that are produced by the guild base. These so called 'spawn NPCs' had a level cap of 30 and respawned automatically at no cost after a fixed period of time, but since it wasn't possible to change their appearances and AI programming, they did not pose much of a threat against other intruding players.

Mercenary NPCs
Mercenary NPCs are entities that players could buy out through use of YGGDRASIL gold.

Custom NPCs
When a guild takes over a base with a castle-level rank, they could create NPCs with a collective maximum level of 700. Since the highest level was 100, you could create a maximum of five level 100 and four level 50 NPCs as an example. When creating an original NPC, in addition to their appearance and AI, it was even possible to change their armors and weapons.

Trivia

 * A guild weapon can alternate changes to a custom NPC's original setting.