Non-Playable Character

Non-Playable Character or NPC for short, is a generic term that refers to entities in DMMO-RPG 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 that existed 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. Prior to fighting Shalltear, Ainz noted that NPCs could not engaged in combat as long as they do not consider the player's action to be a clear sign of hostility.

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 experience 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. In other words, they pop-up automatically from the dungeons. With a maximum level of 30, they have enough power to protect the stronghold. POPs don't need gold to summon, but have maintenance costs depending on their type. It is zero for undead, but living creatures, in particular large creatures, had maintenance costs increasing proportionally to the size of their bodies and could rack up a ruinous maintenance bill for the guild. Nevertheless, they could spawn continuously at no cost after a fixed period of time is over. In contrast to Custom NPCs, Spawn NPCs have no player-made character settings and it isn't possible to change their appearances and AI programming.

Mercenary NPCs
Mercenary NPCs are entities that players could buy out through use of YGGDRASIL gold. It could also be summoned by spending guild finances. According to Maruyama, he stated that some players hire them to make a team. They are vastly inferior to players in terms of AI or equipment.

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. They can be customized freely, with their power level depending on the overall limit of NPC data provided by dungeons or other means like in-game cash. When designing an original NPC, one could used customization tools for AI, levels, weaponry and other equipment. This includes clothing and appearances too. As a result, one could create NPCs that were far stronger than the automatic spawns and place them in key locations. They are particularly made and then used as powerful guardians to safeguard the Guild Base. Of course, that doesn't mean every NPC had to be designed for battle anyways. A guild is totally free to customize its NPCs just for decoration purpose.

Trivia

 * Upon arriving in the New World, it seems that the NPCs of Nazarick are prime examples 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 Item Boxes like players for storing equipment.
 * In one of Maruyama's tweets, he stated that it is usually not possible to make NPCs without a guild base. Though administration-request-type World Items are an exception to this.
 * From one of Maruyama's tweets, he stated that the Guild's restriction over Custom NPC level cap can change by either spending real money in-game, using a World Item, or the Guild Master taking up a special quest, etc.
 * Intruders who invaded the Great Tomb of Nazarick are partly comprises of Mercenary NPCs during the raid.