User blog comment:Maki Kuronami/Lesson 15: Faunus and Robots and Reveals... and Burne and Joel./@comment-15108745-20131101015344/@comment-4010521-20131101041710

perhaps the grimm are similar to the kishin from soul eater, if a faunus does something really bad (like eating a human soul) then they turn into one of the grimm. AND NOBODY IN UNIVERSE KNOWS.