NPCs (non-player characters) are humans who are not controlled by players. There are several different categories of NPCs in Pixelmon, each playing different roles when players interact with them.
All of the above types of NPCs spawn naturally in the world, each having different spawn conditions. NPCs can also be created and edited using an NPC editor, obtainable in Creative mode.
External JSON files
By enabling the "useExternalJSONFiles" config setting, the JSON files used to specify spawning data for NPCs will become editable at "<Minecraft directory>/pixelmon/npcs". More information about these JSON files can be found here.