Aside from Player Characters and the Dungeon Master, Non-Player Characters are one of the most important and necessary ingredients for a game of Dungeons & Dragons. The players need other characters to interact with, rescue, fight, and gather information from — essentially, there wouldn't be much of a game without characters to inhabit the Dungeon Master's world.
Source:https://www.cbr.com/dnd-homebrew-non-player-characters-tips/