Stiles is the younger sibling to Emma and a hunter, and they're in the town looking for their mother who is obviously Snow, and don't yet know she's been turned into a vampire. Also in their home town, Stiles's bff got turned by a vampire after him and Emma left and he got a letter about it and blames himself for not having been there to kill the vampire -- aside from terrible guilt issues, it also presents a moral dilemma for him because before, his approach to vampires was very callous and appropriately hunter-like, so basically we hunt those who hunt us (because the Argent reference just begs to be made). But now he actually knows a vampire and that's his best friend who is an actual angel so it's like ???!?!? so what if other vampires are like that too?? And then SURPRISE their mom is a vampire too... so if anyone wants to have soul-searching discussions about vampire nature or other hunters want to go what the hell get these thoughts out of your head at him, please feel free!
Bucky is a turned vampire (turned before the plot starts) who was already working as Elsa The Mayor's bodyguard (he started out in general security detail or something, coming from a rather poor background, and worked his way up) when he got turned -- incidentally by the same human scientist who turned Art. Or maybe his lab partner or something. (And wow is the idea of Zola and Moral working together a terrifying one.) It happened as follows: the scientist tried to reason with Elsa to get funds for his research, Elsa shot him down because LOL NOT MAKING VAMPIRES ARE U CRAY and so the scientist later on tried to abduct Elsa either for testing or ransom, Bucky did his job and stopped him but got taken instead. He was experimented on, got turned, but that also means he got stronger and managed to escape where he was being held and returned back to Elsa, who should have turned him away but didn't... and that is how the mayor of the town got stuck with a vampire for a bodyguard. The fact is, of course, a well-kept secret, and so far in the know are Elsa and Art, open for others, just poke me about it!
Constance has run away (with vampire hunter d'Artagnan's help) from a loveless marriage after her husband tried to sell her as prey to the vampires living in her hometown. She ends up at this AU's town, homeless and without money, and goes to the church for a place to stay, and there she meets Aramis. She stays there and pays for the lodgings by being a seamstress -- and through Aramis, she meets Anne, who hires her; officially to make dresses for the court, but what she really does is act as the confidante to both Anne and Aramis and runs messages between the two, as Anne can't always make it to the town as often as she'd like, being the queen and all. Her opinion on vampires was very low at first, mostly because the vampires in her hometown sucked, but after meeting Anne (and probably Porthos), she holds a much higher regard for them and so she'll be helping with the secrecy thing in the town, most likely. Of course, this conflicts with her thing with d'Art....
Gwaine is a pureblood who hates all purebloods and the prey system and the way many vampires will treat humans as "lower" in status, and up until a point, his approach to life was akin to "do my own thing, pretend to be human and walk the fuck away to all vampires bye" but then he meets up with The Boyband (see Isil's comment above for details) and while he thinks the whole "free the preys" thing is probably unrealistic, it's still better than how things are now -- and he's great at fighting and likes it (the only thing about being a vampire he actually likes), and it looks like they've got low odds and lots of chances to fight arrogant vampires, so sign him the fuck up. So basically he arrives in the town with Jaeha and whoever else is in their band of hunters, and he intends to free the preys from the vampires they are bonded to! I welcome all the things with him, but especially hunters going "wait what you're a hunter and a vampire wut" or other vampires (especially purebloods) interacting with him, either making him see Not All Nobles Vampires or outright going "you're betraying our kind!!" at him.