The Stump's Spren
A Truthwatcherspren, and so a mistspren, appearing to the Stump like light reflected off a mirror onto a wall.
The Stump's spren is a Truthwatcherspren, which is to say a mistspren. The Stump describes her as looking like light reflected onto a wall from a mirror, and a scribe puts it as the shimmer thrown on a wall when light is shone through a crystal. Mistspren choose their own forms and usually settle on something shaped like a person, though nothing obliges them to. In her free shape she strikes Shallan as difficult to capture in a sketch, something like steam somehow trapped into a humanoid outline and held in by clothing and a strange mask.
How far into The Cosmere have you read?
Set your place and this page rewrites itself to exactly that point. Nothing ahead of you will show.