It was Metatron, he was able to erase the markings, get in, and save Kevin. Right after this, he even says I saved him directly.
I know, but the question is, was something happening before Metatron arrived? It was very strange that Kevin suddenly had no fear of Crowley-- and the light came from Kevin's eyes.
There was speculation that Meg wasn't actually killed by Crowley in her appearance this season. She seemed to be genuinely in love with Cas, so I think she's half way there. I would bet she's our demon.
Meg was my first thought, in terms of wishful thinking. Crowley is the obvious choice, but I doubt if the showrunners would give up their Doctor Doom.