I actually heard about her being a demon (I don't really care for the rest of the series, I was just interested in this turn of events) and thus I'm quite glad that there's already fanart of it.
Aaah, Gainax. Ending every season with a completely incomprehensible ending so that they can do pretty much whatever they want with the possible following season. I wouldn't be surprised if, in season two, they explain away that line as rubbish in the very first episode.