The guy on the cover is definitely Adrian. See Richele post on her blog:
If you missed the Bloodlines cover and accompanying video, you can check out Entertainment Weekly's article right here. I'm also using the cover as my avatar today, and to answer a few questions, yup--that's Sydney in there. You can spot her by the tattoo. The guy is the spurned love interest from Last Sacrifice, which I'm being enigmatic about simply because talking too much about him gives away the ending of that book. There are still lots who haven't read it, so we'll hold out a little longer. There are other characters in Bloodlines that I've mentioned before, like Eddie and Jill, but if you crowd too many on the cover there, it starts to look like an Olan Mills family photo shoot. (Are they even around anymore?). So, we just kept these two, but other characters will rotate through on other covers, I'm sure.
However, it doesn't mean Adrian and Sydney will be love interests. They maybe the primary focus of this book. The next covers could have Jill or Eddie on it.