So! Who the hell are we talking about, anyway? Bella Swan and Edward Cullen may be the most famous fictional couple of the century, the two main stars of Twilight--a human and a vampire. Buffy Summers and Angel are another famous fictional coupling of human (though superpowered, as a Slayer) and vampire. Cassie Hughes and Azazeal are a bit less famous (though likely better known in the UK and Europe, since Hex was a British show), and consist of Cassie the girl and Azazeal
the fallen angel or Nephilim. For reference sake, Buffy aired from 1997 until 2003, Hex aired from 2004 'til 2005, and the eponymous first Twilight book was released in 2005. There is, obviously, a good deal of overlap in these time frames and generally it can be agreed on that the mid-to-late '90s and early '00s were when the vampire craze really boomed. We're still in the middle of that craze, with books, movies, and TV shows being released or optioned or piloted from here to Hell (sometimes literally). Vampires are sexy.
Cassie, Bella, and Buffy agree with you! No worries there. True, Azazeal is not quite a vampire, but his powers, attitude, and motivations are similar enough to those of Edward and Angel that he can be grouped among them. Some facts about the two major players who AREN'T that well known:
Azazeal: Azazeal is technically older than written history, being a "Nephilim" predating civilization. In Hex's mythology, the Nephilim are angels who fell in love with mortal women and were cast out of Heaven (note: in the Apocrypha, where the Nephilim are mentioned IRL, they are classed as being the children of mortal women and Grigori, "Watcher" angels. Are you laughing yet?). As such, Azazeal has been seducing mortal women since before history was written--his earliest recorded actions date to ancient Egypt. His true face is horrific and demonic, but the face usually seen is far more pleasant and deceptively human. He is capable of possessing humans, during which they are easily bent to his will, has greater-than-average strength, can teleport, and can cast spells. He does not generally even make a pretense of being not-evil.
Cassie Hughes: never stated outright, we can assume that Cassie is between 16 and 18 years of age based on her presence at Medenham School, a college in Britain. She is relatively shy and unpopular, with her closest real friend being Thelma Bates, an outgoing lesbian girl. Cassie is talented at art and an all-around good student. She discovers that her ancestress Rachel McBain was a witch who lived at Medenham before it became a school, and that she too has powers in witchcraft, including telekinesis and flashbacks of the past. She is blonde and quite pretty. Cassie never intends to use her power for ill and, despite being possessed and seduced by Azazeal and maintaining a certain sympathy for him, she is not evil.
And these are the folks we'll be talking about. Keep in mind--Hex and Buffy are television shows (the Buffy Season Eight comic will not figure in this discussion) and the Twilight series were books before they were movies. I may refer to the film versions of the Twilight series--damn, for the first time I actually wish Breaking Dawn would hurry up and get released--but I prefer to concentrate on what the author wrote, rather than how a series of filmmakers interpreted it. Until next time!
Note: More information about character ages, biographies, and so forth, as well as these images, may be found at the following sources: the Buffy/Angel Wiki, the Twilight section of Meyer's website, and the Hex Wiki.