Randomized NPCs are townies that usually appear on the Main Streets of islands. On the reload of the scene, the previous randomized character is replaced with a completely different one, while keeping the same dialogue.
On reload of the scene, a randomized NPC will completely be remodeled, while keeping the same dialogue. They could go from female to male, long to short hair, etc. They aren't supposed to be the same character each time - it creates the illusion of many inhabitants.
The randomized NPC's barely ever have anything to do with the plot. However, there's been some occasions like Spy Island where their hair gets zapped away while they each say different things.