HadesThe Lord of the Underworld. He's the head of the Underworld, and is a very autocratic ruler.
PersephoneThe Queen of the Underworld, and a Goddess of Spring. It seems rather odd doesn't? Death and Spring...One of the two august Goddesses.
HecateThe Witch-Queen! Goddess of Magic and Witch-craft. She's quite powerful, and can grant mortals anything she wants to grant them. Definately a Goddess to curry favor with.
Zagreus The God whom the cult of Orphism is based on. The son of Zeus and Persephone. After reading about him, read about Adonis' birth to prove that his parentage isn't all that odd in Greek mythology. (And they tormented Oedipus!)
The Gods' Loves Adonis, Leuce, and Minthe! Oh my. Aren't you curious about who Persephone and Hades felt lust for in mythology?
The Helpers of Hades Cerebrus, Charon, Ascalapus, Meneotes, and Hermes. All of these people/creatures/Gods make things far easier for Hades. An Underworld is a lot of work to manage even for a powerful God.
Sleep, Death, and Dreams Hypnos, Thanatos, and Morpheus. Very romantic, of course. The pre-Raphelites loved them....scary.
The DamnedStaring Sisyphus, Tantalos, Ixion, and the Danaids. What do you have to do to be tormented in the Underworld? How does Hades (and the Judges) choose to torment people? And how did Sisyphus trick Hades, who is usually so careful about souls, into extending his life?
The JudgesThese sons of Zeus determine the fate of all those who die. Minos is a fairly intersting character. Did you know that his daughter married Dionysus?
The Controllers of Fate The Fates, the Keres, the Erinnyes (Furies) and Nemesis. These are the creatures that control your end...beware!