I lean toward Demeter, since it would be more of a surprise reveal. We think of her as the goddess of growth- but her mourning for Persephone each year caused winter and death and misery to which she was indifferent.
From a Guardians of the Gates standpoint, it's been the Fae for at least a millenium. A millenium gone, the Norse gods were at their peak for worship. A millenium before that, the Roman gods; a millenium before that is questionable, but the Greeks nearly align, especially with how Jim plays with his mantles. A millenium before that- Egypt, maybe? A millenium before that, roughly, Sumer, with their mythology of order coming from the slaying of the primordial serpent. Before that, ?- dragons, maybe. As members of the Accords, we've seen Sumerian-speaking ghouls, Etruscan speaking vampires, old Norse gods, and in the background a Greek/Roman deity (is Hades a signatory?).
No real point, just tossing things out there. Millenarian movements are a very real force in human psychology, and Jim seems to be hellbent on creating an "Everything is in it!" series.