Board Thread:General Discussion/@comment-59.101.166.234-20150901053929/@comment-4010415-20150901060834

Everything that SpiritedDreaming said, though that This Will Be The Day line was kind of misquoted. The "bloody" part is in "Welcome to a world of bloody evolution," which, fun fact, was originally going to be "Welcome to a world of endless evolution" as evidenced by the early version of the song playing during the volume 1 opening storyboard easter egg.

Also, Cinder uses Dust. According to World of Remnant: Dust, Dust is activated by the Aura of humans and Faunus. Cinder's main weapon is raw Dust, which she controls with her Aura.

Grimm do not have Aura. Aura comes from the soul, and Grimm don't have souls.

Therefore, Cinder can't possibly be a Grimm with a humanoid form because it can't be a Grimm if it has a soul.