Supernatural Geists

Now, I don't have the book yet, might be picking it up today.

I'm not exactly sure what a Geist is, but is it possible for a supernatural creature's ghost to become bound to a Sin-Eater?

If a Supernatural creature is capable of leaving a ghost behind when it dies I don't see why not.

However Geists are not really ghosts anymore.

Kruhnk:

I'm not exactly sure what a Geist is, but is it possible for a supernatural creature's ghost to become bound to a Sin-Eater?

A geist is described as being something more than a ghost and more like a minor death god.  It's similar in some ways to a spirit of the Shadow Realm in that it's as much a representation of a concept -- an aspect or form of death -- as it is the lingering spirit of a mortal being.

A geist could have originally been, in life, some form of supernatural being, but it wouldn't really matter, for two reasons.  First, the ghost of a supernatural being is still a ghost.  It may retain some of the powers it had in life (or undeath, in the case of Vampires) as Numina, but they're simply ghostly echoes of those original powers.  So the ghost of a vampire is simply a ghost with some interesting Numina.  Second, in becoming a geist, a ghost undergoes fundamental changes that distance it from ordinary ghosts.  Their powers are far beyond those of ghosts, even those of supernatural beings.  But that still doesn't matter, because the powers a mortal gains in merging with a geist in becoming a Sin-Eater are the same regardless of the geist's origins, and are determined by his Keys and Manefestations.

So yeah, if you want to say that a particular Sin-Eater's geist was originally a Mage, go for it.  It won't provide any mechanical advantage, but there's nothing that says a geist couldn't have once been a Mage.

And of course since ghosts flock to Sin-Eaters, it's possible that the ghost of a Vampire/Mage/whatever might seek out a Sin-Eater, but as I said, it wouldn't be any different from a normal ghost except for its unusual Numina.

Yep - its possible.   The process of transforming a ghost into a geist sheds most of the original personality, memories, and abilities, though.    Geist are the distilled essence of death (think of them as death elementals); anything mage- or vamp-like about the geist had been rubbed out.   There's next to nothing human-like about these geist left.   All you have is some memories and personality inclinations left.   

Also note that Geist don't appear to be human.  Or at least they usually don't.   There are some that appear as a rip in the fabric of reality, for instance.

Despite not being especially mechanically different (As everyone has said), there IS an advantage to allowing once-Supernaturla Geists.  If you do that approach, someone can actually bring back one of their dead characters.  If you once had a vampire that would seduce people and then drain them, in death he may have become the Bloodletting Lover, who embodies ALL kinda of murderous seduction, death at the climax of great pleasure.  He'd probably have the Stigmata Key to give him some of that blood kink, and maybe Passion for the seduction itself.

Similarly, if your mage was a Forces master and was blown up, the Burning Wizard could be appropriate, with Pyre-Flame Key and an especially philosophical leaning.

Of course, as everyone has said, a good portion of the original personality is whiped away by the process.  Geists typically remember their Death fairly well (Though, that is not a universal thing, as many Geists don't remember how they died specifically, just how they want to portray death).  Elements of quirks and personality will be there, but will not seem to have a cause (IE a person that used to look in the mirror because of poor self esteem might glance into any mirror he passes, despite the fact that his head is now an empy crater left in a skull).  A few Geists DO remember bits and pieces, even their original name.  However, GEists tend to be incoherent and vague until they are bound as a Krewe Founder.