User blog comment:Dturpin48/Eve/@comment-5120709-20120520070048

She is not a Leviathan.

If you remember in Season Six they mention Purgatory has some species native to it, like that woman whose names escapes me right now who dated Bobby and was killed by Cas. And speaking of that woman remember how she was killed by Cas, while Edgar was immune to the Angel's powers.

So my guess is that Eve is just one of the natives of Purgatory with some relation to the Leviathan, like a branch off evolution or so