NPC's can range from anything, even if they aren't technically units. Many characters in the Star Wars universe appear in the games but don't have any interaction sequences. This can include characters like Jabba and droids victim to torture in some maps.
Other NPC's appear that have special interactions with the player. Droids such as the Health and Ammo droids cannot be played or moved, but will interact with the player by giving them their respected items.