Prologue: Light
Even the lightest of creatures have, somewhere, a speck of black under their skin. When the black is so large, it spreads past your blood and to your skin, thus painting your skin black. These black creatures are called Grimm. Nobody knows- or knew, that is- how they showed up on this world, millenias before the appearance of humans. Light has shown up in the dark, and we fight for this to continue. But we never will truly be the pure white we think we are.
Some creatures that are not quite at the stage of a Grimm are so dark that their insides are dark with evil, and their outsides are the only good. These can be animals, people, anything. There's no way of telling one when you meet them, for oftentimes they hide their inner thoughts, and sometimes you can know one for years before realizing that they are not as pure as you had thought them to be.
Sometimes you can know them for a lifetime, and never realize that their skin was the only light they had.
And, even less often, you may find a creature with an evil, shadow-like skin, but somehow it never got to their heart and they're good creatures on the inside. You can never be sure.
The shadow-skinned Grimms can be human at heart... and a human can have a black soul.