Settler (Fallout 4)

A settler is any person who decides to join a settlement. They can be found all over, especially near settlements.

Settlers in allied settlements
Speaking to a settler from one of your settlements will allow you to access their inventory and select their equipment much like you would a companion. They can be armed with better weaponry than their pipe pistols, and they don't seem to consume ammunition. All the same, they only need to have one unit of ammunition of the type their gun uses in their inventory to be able to fire their weapon. Then, they can fire at will and not consume any ammunition (confirmed by multiple people here: www.gamefaqs.com/boards/164592-fallout-4/72809150). This does not work for the missile launcher or Fat Man, as these are tagged with "NPCs Consume Ammo". The same holds true with grenades.

Settlers assigned as shopkeepers have additional dialogue options when spoken to similar to other merchants. Select "trade" from the dialog tree to access their inventory. Speaking to a shopkeeper settler outside of their operating hours will go straight to their personal inventory as normal. Doctors seem to be an exception. A settler assigned to a doctor stand will not have a trade dialogue option when spoken to during business hours. To access their inventory, either wait for the shop to close or unassign and reassign them from/to their shop.

Appearances
Settlers appear only in Fallout 4.