I know you said you checked Wikipedia, but surely [link] couldn't really lay it out any easier? Just look through the list of immortals and find a female one that suits your needs. When you have one, research her individually some more. Failing that, look at the underworld itself [link] and see about what was actually meant to reside there.
If there are no Greek figures which work, you have two options: make one up or change the character. I wouldn't recommend making one up, because (in my very limited knowledge, admittedly) the series uses the deities from mythology rather than making them up. Besides, the Greek Gods were pretty messed up regardless of their relation to the underworld, so it's not really a necessary connection if you want to have a messed up child.
Oh, ok. I've looked on Wikipedia, and read through a bunch of names and descriptions, but I can't find what I'm looking for. Ideas for any other places where I could look would be greatly appreciated!!!