Zero will be as if his family was never attacked by Shizuka, therefore him never having been turned into a vampire. So, a hunter through and through, while still very much a prodigy. Which means he won't know much about Yuuki, however definitely be familiar with the Kuran name, seeing how they're the purebloods who lead the vampire world. All that aside-! Zero will be more like he was when he was younger, so. Still ultimately very kind, while also more openly friendly. Rather than more subdued, and still with traces of being angsty and serious.
Jae-Ha is going to be a vampire (from the Vampire Knight world, of course), because I've always wanted this AU! He will be one of the noble class vampires, but rebellious rather than proper and, well, noble-like. Back "home" he'll have been known as a troublemaker, always toeing the line just so far that he's never actually been detained or listed for being executed. — Also, aside from the usual inhuman strength/speed/reflexes, heightened senses, etc etc, his special ability is air manipulation.
. . .Otherwise, his personality will be very much the same. Playful, teasing, perverted, ridiculous, seductive, confident, etc. It'll be great.
Kazuki will be of the clichéd coffee shop AU! Because he deserves something without angst and death. At home he will have been an awesome barista (while of course also a skilled cook!), as well as a university student, who's also a talented athlete and fighter.
All three will be as though they've been in Marina all along. I also plan to toss up a bit of info about each character's AU in their journal for quick reference and all.