The answer here is skeletal.
That is because Skeletal also has as meaning "very thin; emaciated.". That is the case because when someone is overly skinny, you can clearly see their bones curving their skins. Many times, the shape of the bones is so clear that people will be reminded of skeletons immediately. That's why Skeletal might refer strictly to someone's appearance when it comes to be about "someone who is so skinny that you can almost see their bones".