Been a while since I haven’t drawn Double’s true form in general-
For those who don’t remember, she’s a member of an alien species known as Ondinians, they’re known for their ability to shapeshift into any Water-type Pokémon, especially when they can separate their limbs so that they can all shapeshift into different individual ‘mons.
Another thing is that her species lack bones, instead relying on soft cartilages on their non-transparent parts, but the separatable parts (the transparent ones) are made of some kind of protective gelatin that surrounds the nerves.