Soooo, this is from Adri's canon universe. He's not actually a demon, but it's easier to say "demon" than "genetically advanced specimen", so yah. *SHOT*
This is by no means an original or awesome design, but I prefer something simple for this. :I
Basically, Adri's dad was as a teenager put through a series of genetic experiments (willingly, I should add) which gave him the ability to manipulate a tar-like substance. However, he himself couldn't turn into that substance. The closest he's come to doing that is to engulf his body in matter from outside, which isn't the same thing. He's basically wearing a suit, and that doesn't count. :I
However, previously these gene-manipulated people have never had their abilities carry on to the next generation. But Adrian inherited his father's abilities and more, and he already started to develop these abilities at the age of ten.
He can at will change his entire body's molecular structure and become this black "demon", and in this state he's difficult to harm since he can just let weapons and other objects pass through him as if he was made of liquid. His weakness would be his personality, which remains the same in whichever form he's in. He's still very kind, caring and honest, and he can be somewhat gullible and naive.
He does speak in his demon form, but his voice is distorted and it somewhat sounds as if two versions of him are speaking at the same time.____________
Adri & art (c)