A Non-Player Character (NPC) is typically a character that is not controlled by a player. They are often seen serving limited functions, such as shopkeepers and town guards, and can be used to further or assist role-play.
Despite the term 'Non-Player Character', players can have NPCs as well. Such examples are a noblewoman with an NPC bodyguard, a farmer with an NPC dog, or an NPC villain meant to entertain players with a storyline. NPCs tend to be less rounded in personality and serve a limited purpose. They are either displayed as an attachment to one's avatar, represented as the avatar without a player-character, or not displayed at all (but mentioned in a titler so others are aware of the NPC's presence).
A character may only have one NPC at a time. This is to prevent people from making posses or armies of their own. If you wish to have your character be accompanied by several people, please seek out actual player-characters to accomplish this.
Only the owner of the NPC can role-play for that NPC; its actions cannot be presumed by others, unless it is an NPC that belongs to the sim (such as RPing with a shopkeep set up by the sim). All NPCs that are player-owned are held just as accountable as player-characters are. If you use your NPC to commit a crime, please make them accessible for the consequences. Having an NPC means they are open to attack like anyone else, but only the owner can decide to let their NPC die.
In combat, only one can fight--either the player-character or its NPC. Both share the same HP; if one reaches 0 HP, both are incapacitated.