Butter is a Human Female that hangs around Griff's Kitchen, she is also the Huntsman vendor for the island of Fort Joy. You will find her wandering between the Shrine of the Seven and Griff's Camp.
It should be made clear that she is a part of Griff's crew and will attack if you start a fight with Griff, meaning if you kill her there will no longer be a Huntsman vendor on the island.
Butter's inventory consists of...
Arrow Spray, First Aid, Mark, Marksman's Fang, Ricochet, Snipe and Tactical Retreat.