Willa is a strange little creature, small and somewhat round.  However her podgy shape is probably formed more of air than of fat, as she inflates herself to achieve a 'normal (small) body size'.  At a moments notice she can expel the air within and deflate to a much tinier, thinner version of herself.

Her entire orange-ish body is slimy and translucent, making her glow when in front of a strong light-source.

She has a chubby, childlike face with large, golden eyes and wide smile.  A flop of jelly hangs over her head like a fringe.  However her head is now often hidden behind a large intimidating helm.  Its visor forged to look like a sinister skull, and its brow adorned with gems that sometimes glow.  When she lifts the visor or removes the helm, Willa's malleable face retains the scary shape for a few moments before popping back into the usual cheery puffball.

She appears to glide as she moves, travelling at a far greater rate than her waddling would imply.  As she does so, momentum causes slight warps and ripples across her skin