Here’s some back story that I think will only ever be implied in the comic: All the gods are real. Every one. From the Greek and Roman gods to the Native American gods, every one exists. At some point, certain gods became too weak to keep their roles, as the people that worshiped them died or forgot them.
Eventually, the smaller and lesser know gods and goddesses, such as Manea, were dethroned and cast into the mortal realm, while the “super powers” (Ancient Greek, Norse, Major Hindu, etc.) developed an unwritten and uneasy ceasefire, meaning that Hades, Osiris, and a handful of other gods rule the underworld. (Satan rules Tartarus, which was fine with basically everyone, because he also has to live there.)
As Manea proves, though, the small gods didn’t just disappear, and she still retains her powers.