User blog comment:B214/Question and Answers/@comment-24098614-20131121080854/@comment-1359322-20131121095535

I don't think it's possible for an Angel to become a Devil even if it's pure-minded. Like in Volume 12, top level Angels aligned with Khaos Brigade were able to avoid falling because they were in Heaven and a loophole in the system but fell immediately after leaving Heaven. So by any means if an Angel has evil or negative thoughts, they would fall immediately after leaving Heaven, yeah being a Devil doesn't mean you need to have evil thoughts but remember Holy and Demonic are of opposite nature, the Angel will fall once they accept demonic energy.

The Yuuto's case is like an exception because it was made possible by combining his Sacred Gear with the light attribute in humans (yeah they were dead but they were humans). But i won't deny any chances of there being an Angel/Devil hybrid. But then again it might take a long time maybe after Michael and Irina successfully develop a way to do so.