Ethan is a guardian (essentially humans turned angels) who has been keeping an eye on Charlie. Charlie is an ex-singer who is struggling to recover her life after a self-destructive streak. She's being watched by the guardians because her sister is currently dating a demon who wants to find a cure for vampires' bloodlust. Unfortunately, the demon has made plans for Charlie in order to secure her sister's cooperation.
Honestly, I wasn't too enamored with Demon Night. The storyline was good, but it dragged in some places. I also wasn't really able to get on board with Charlie and Ethan's relationship. Ethan by himself was great and Charlie was an interesting heroine, but something about them together just didn't click. The side characters really made up for the lack-luster relationship though.
We see a lot of Jake, a young guardian who was a Vietnam vet before being changed. He offers a lot of humor to story and it's great to see the way he interacts with Charlie. I also enjoyed how Brook used this plot to muddy the lines between good and evil. The first two novels in the Guardian series really have that "demons are pure evil" vibe going on. Even Demon Angel does despite having a demon heroine, because her "goodness" is subtly attributed to her being half demon. Demon Moon really starts to blur those lines through showing the relationship between Charlie's sister and her demon boyfriend. Yes, he has an agenda but it comes across that he really does love his girlfriend. However, the sister's attitude to what he does to Charlie pissed me off. Then again, the sister in general annoyed me.
All in all, this is a really great paranormal series, but if you're going to start reading these books, I would suggest doing it in order.