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. To do so however, some of these NPCs required financial means like the mercenary system existing in guild bases. 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. They were nothing more than a collection of electronic data, and the closest they could come to real emotion was a very skilled set of inflexible AI routines. Because of the way NPCs were set up, guild masters were given the privilege to speak with NPCs and have them carry out tasks through specific commands.

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. They're able to adopt the creators' manners and beliefs, which were not covered by their character details, as well as in the interpersonal relationships with other NPCs.

Reasonably so, 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 and things they say. This would include as well as the misunderstanding their creator's way of life, words taken out of context literally, etc. Momonga had managed to learn from his interaction with the likes of his NPC, Pandora's Actor that their knowledge is affected by their character settings, but they could ignore any contradictions in their backstories.

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. Unlike players, Custom NPCs could also be resurrected without losing any levels, but can cost an unknown amount of YGGDRASIL gold to have such action be carry out by the guild master.

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
If a guild possessed a castle-grade guild base, the owning guild would be allowed 700 levels to be distributed between any number of NPCs they wanted. Because the highest level in YGGDRASIL was 100, by those stipulations, a guild could make 5 level 100 NPCs and 4 level 50 NPCs, or any combination thereof. When designing an original NPC, one could customize weaponry and other equipment in addition to clothing and appearances. As a result, one could create NPCs that were far stronger than the automatic spawns and place them in key locations. Of course, that doesn't mean every NPC had to be designed for battle anyways.

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 is a prime example of taking on their creators' nature. More so, remembering the words of what their creators said in the past as important to them.
 * According to Momonga, the NPCs should have inventory pocket spaces like Ainz did for storing equipment.