Illya is a character created by ShadyShadsie.
Her regular form is often regarded as an anthro/humanoid Polteageist, although in what seems to be Illya's canon, both her ghostly and slimy properties allow for a complete shape-shifting experience, including colors, scents, textures, temperature and the emulation of hardness to some point despite her liquid-like body. These shape-shifting properties can be often granted temporarily to other inanimate objects and even living beings, although it is most of the time not controllable by those who are experiencing such changes.

She's usually seen in her original, female form, but that doesn't change the fact that she can pretty much be anything or anyone at all times, kinda like a Ditto