Lynsay Sands has created some really interesting characters in this series. One popular character is Argeneau himself. He's a complex vampire with a long history. Another is his love interest, whose name I can't quite recall at the moment but she brings a lot of humanity to the story.
One popular story could be about a new vampire in the Argeneau world trying to adapt to modern society while hiding their true nature. Another might focus on a forbidden love between an Argeneau vampire and a human from a rival vampire - hunting family.
Lukian Argeneau is often a favorite. His strong and mysterious nature makes him a great character to build fanfiction around.
The Argeneau series novels often touch on the theme of power as well. There are different levels of power among the vampires, whether it's physical strength or political influence within their vampire society. Also, the struggle between good and evil is a theme, as some vampires may use their powers for less noble purposes while others strive to protect and do good.
I'm not sure specifically as I haven't read it yet. But usually in an Argeneau novel, there are members of the Argeneau family which are likely to be main characters.
Not having read the book, it's difficult to name the exact main characters. But we can assume that the titular immortal angel is a main character. There could be other important characters such as a mentor - like figure who guides the angel in the ways of the Argeneau world, or a sidekick - type character who helps the angel on their adventures. There might also be a villainous character who is envious of the angel's immortality or has some sort of vendetta against them within the context of the Argeneau story.
I'm not sure exactly as I haven't read it yet, but typically in an Argeneau vampire novel, there would be members of the Argeneau vampire family. There might be the runaway vampire as a main character, and perhaps a love interest, either human or another vampire.
Typically, in 'Under a Vampire Moon An Argeneau Novel', there will be at least one central vampire character from the Argeneau family. This character could be a centuries - old vampire with a complex personality. There might also be a human character who gets involved with the vampire, perhaps a woman who is initially unaware of the vampire's true nature. Their interactions and the development of their relationship would be a key part of the story. Additionally, there could be other vampires in the family who play important roles, like an older and wiser vampire who gives advice or a rival vampire who causes trouble.
I don't know for sure as I haven't read this particular novel. But in Argeneau novels, there are usually characters from the Argeneau vampire family. So there might be some members of that family as main characters in this one too.
There's likely an Argeneau vampire who is a key character. This vampire might have some special traits or abilities within the context of the Argeneau vampire lore. The love interest is also a main character, and they could bring a different perspective, perhaps a more human or non - vampire view. And there may be an antagonist, perhaps another vampire or a force that opposes the relationship between the two main characters.
There could be a wise old mentor figure. Maybe someone named Guru, who imparts knowledge and guidance to the younger characters. He would be the one with deep understanding of the world and its mysteries in the agniputra series novels.