Many of the Non Player Characters are non-essential to the main storyline, however most of them have small quests or functions of their own.
Some of the NPCs in the game will make themselves available to the player at bosses as helpful phantoms if the player is in human form.
Conversely, there are also NPCs that will also invade the player's world through scripted events.
Note: Black Phantoms will only spawn if the player is in human form and the area boss has not been defeated.
Therefore if the area boss is defeated before being invaded by a particular NPC Black Phantom, that NPC will not become available until the next playthrough.