There aren't many pure blood demons left on Earth. They use to have control over everything, but eventually they lost their hold. Demons that are present now are varying and different. There's different races. Some can look human, some look like giant snakes, or just massive things. Different breeds all have different appearances, so I can't really describe how they look unless you have forever to spare. I'm sure don't know all the races out there. I've fought hundreds.
They can be people turned into demons, creatures from other dimensions, and more. Some are decedents of the Old Ones. Some are more violent than others, some are just trying to stay hidden and live their lives, not all of them deserve to be sent to hell.