Non-Playable Character

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

Background
In YGGDRASIL, NPCs exist within the game to provide support to the players and aid them in battle through financial means. Their purpose was to deliver quests, act as support characters, etc. On the other hand, players who're usually in a guild, controlling different dungeons, and cities can also customize and regulate their NPCs to certain positions over an area specifically designated for them only. In particular, the Custom NPCs are created with different races and classes likewise to how players create their character, but to serve a role in the field they are most fitted to be at.

Moreover, 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 undergone a miraculous change. Unlike how they had been when they were merely data, the NPCs now had minds and the capacity for independent thought. 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.

Because of that, Ainz had learned that the NPCs’ knowledge was affected by their settings, but they could ignore any contradictions in their backstories. The NPCs seemed to take after the former guild members from Ainz Ooal Gown in the parts of their personalities which were not covered by their character details. In addition, as well as in the interpersonal relationships with the other NPCs living in the Great Tomb of Nazarick too. For instance, the relationship between Shalltear and Aura, or that between Demiurge and Sebas.

Moreover, they can established either new relationships romantically or new connections politically outside Nazarick and into the New World with their native inhabitants. In particular, a prime example would also be Sebas and Tuare who likes him or Demiurge, Albedo and Renner, an accomplice of theirs in the Re-Estize Kingdom.

Still, NPCs who are created by players were also able to be infused with knowledge upon their creation, but that was all they ever had. Because the NPCs did not know the players in real life, mishaps may sometimes occurred; like say, making a mistake about their revered masters' jobs. This would include as well as the misunderstanding their creator's way of life, words taken out of context literally, etc.

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.
 * Upon arriving in the new world, it seems that the NPCs of Nazarick tend to take on their creators' nature as well. More so, remembering the words of what the 41 Supreme Beings said as important to them.