Three out of ten introduced in the first season, Taiga recurring (possibly regular) and Isaac one of the Scoobies. I think it's quite safe to assume from this that most of us will be Scoobies.
(Note also the title of the RPG, which is a nod to "Welcome to the Hellmouth", the series premiere).

If that is the case, we can't look for alignment in the series, same as in earlier games, and as the bitca (no Yuna, I looked it up) up there pointed out with her mind alteration comment.



Safe to assume that Taiga is vampire, especially when taking into account the conclusion earlier that we might all be Scoobies.
He could also be a demon, in which case I whould vote for him, despite the mind alteration comment.




Isaac could be everybody on the show (I really start to regret my suggestion for a question), although I could interpret it as not under too heavy makeup (i.e.: no demon).
Yuna's possibly Xander or Oz. Riley and some others might fit the physical description (from what I see on his picture), but the childish part points at (what I remember from) the first two.
?: well, plenty of female humans to choose from.

The first season scoobies are Buffy, Xander, Willow, Giles, Cordelia and Angel. So those, plus some others?

I can make some guesses as to who's who, as I can imagine others can too, but the important conclusion is: for now no obvious persons to focus on.