There are several NPCs (Non-Player Characters) that you are able to interact with on the sim.
They provide the following function:
Quest Triggers - They have the ability to start certain quests that you can enjoy for shards, experience, new classes, among other rewards.
Feature Unlocks - NPCs are the standard way to unlock new character features such as the ability to become a shapeshifter sub-class, crafting and gathering profession, among others!
Class Unlocks - NPCs are also the main means to unlock other classes within Spira. They are either portrayed as animesh models, or special objects within the sim.