The smees are doppelgänger extraordinaire, with extremely potent shape shifting abilities. The only record where their disguise is penetrated (without mistake on the smee`s part) was on Walpurgisnacht by Astaroth.
In their native shape, they have been described to slightly resemble a large yam. The smees has no visible eyes, limbs, or any other features for that matter. However, they can take on various shapes, which makes them quite hard to describe exactly what a smee will look like on your first encounter. It is nearly impossible to distinguish a smee from the actual being they are impersonating.
They grow as a grub in the dirt until rain fall washes them away (as Cooper states). Smees are thought to shapeshift because they are shameful of their natural shape.
A smee`s most notable ability is their ability to shape shift, mimicking the mannerisms, aura, speech, and scent of the one they morph into. One smee, Toby, was even able to fool Madr`aast, and possibly even Prusias.