User blog comment:Marco the Marine Admiral/Theory: Evilness Can Lead to Power-up for Dragon Slayers/@comment-4829046-20140628160016/@comment-4829046-20140628160848

Still doesn't give off anything near the original DF as Zero pointed out before. If Brain really trained Cobra to use DF, then Zero wouldn't have given off such a speech. He would've made a reference to Cobra and stated that Natsu was on a different level.