I also think the side characters could've been fleshed out more, to me they just felt flat and like they were only there to help further the plot. I like it in books when every character feels real. Like, most of Jennifer L Armentrout books. She knows how to truly make them feel real. I'd like to hope that as the series went on the side characters get there chance to grow as well. I do really miss books about Fallen Angels/Demons, there used to be a lot more now I feel they are fading away.
Now, this book isn't the best around, there are far better ones out there but it was enjoyable, I found it hard to put down even though it was very predictable and nothing really seemed original. If you haven't read this book yet and are looking for a quick read. I'd say give this a shot.