User blog comment:ProjectPredacon/Grimm from Dust?/@comment-3146930-20150112174046/@comment-25958301-20150112193200

Well, it's actually Aura/Semblance that Grimm cannot use. Aura is the manifestation of one's soul, and because Grimm are soulless, they can't use Aura. It was a nice theory I had though, so who knows.