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Intro. Severus Snape is a Grimm, a person, being with ancestors that hunted and killed, often through beheading, Wesens. A Wesen is a creature that can shift into a human and back into a humanoid animal of some kind. They originate from Germany, hence their German names. Only a Grimm can see a Wesen when they don't want to be seen, and a Wesen can be seen by other Wesens and humans when they WANT to be seen.

Severus Snape [Grimm]

@Laona S. Vega