A skinwalker is a creature that can wear the skin of other people and become them. It is also a name for a trouble.
Skinwalkers were introduced in season 3 of Haven when a skinwalker comes to Haven killing people in order to figure out where the Colorado Kid is. The identity was later revealed to be the Bolt-Gun Killer. The killer killed several people, taking their identities. Roselyn, Grady and Tommy Bowen were victims and their identities stolen. In Last Goodbyes, it was revealed that Claire's identity was stolen as well by the skinwalker.