User blog comment:Sunako30/Who is your top five strongest characters in owari no seraph in your opinion/@comment-33177490-20160102170840/@comment-25942629-20160102234742

Apparently, she and Shinoa were born with demons in them, so she was already becoming a demon from the day she was born.

It's still hard to say if Mahiru is in complete control or she's just turning on Guren's demonic side and influencing that. Right now, Guren himself is completely unconscious, so it's not demonic Guren but rather Mahiru. In the LNs, people possessed by demons still talk and act like they normally do--just the evil version. Their desires mature as they spend time with the demons, too.