Just finished reading Vol 2 and the parts of Vol 3 as wel as parts of 1(Already read those parts in the manga) I could read, and i'm actually pissed now. I was before, after reading the manga pages but, I'm even more now
The difference between Anime and Manga actually having a great impact on characters like Yuuma, at least in terms of appearance... (She didn't magical girl transform into a fallen angel with revealing clothes but just spread wings instead)
As for the Yuuma-Issei relationship:
You will find out later.
I really hope it will get a second season though. The story development is, to describe it with one word, awesome.