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Backstory 1

As the present ruler of the River of the Underworld, she cares greatly about her status as the Fallen Angel and the mention of it can enrage her.
She's moody and unpredictable. The ghosts of the Underworld are often severely punished for offending her with even a small trifle.

Backstory 2

She speaks a patrician language that is close to extinction.
No one understands it except for herself and some deliverers.
It seems she doesn't care about her unintelligible way of speaking, but rather sees it as a sign of her own nobility.

Backstory 3

She likes singing by the River of the Underworld.
It is said that all the ghosts will gather and bow down before her, and she will condescend to sing in an intelligible language for those she sees as inferiors.
It is said that her only friend is Sher, the Wanderer of Fate.
Acheronte admits that Sher is the only one willing to hear her singing and she once said Sher is not a real name.
Sher has a higher status than Acheronte in the Underworld, which is quite confusing for other people to understand. What sort of being in the Underworld can be even nobler than the present ruler?

Backstory 4

She will use honorifics only when referring to Sher.
Otherwise, she always uses unintelligible words when speaking to others.
"O fragile beings, beest grateful to mine own chanting f'r thee."
"Thee crosseth the Lethe and stayeth in hell to atone f'r thy sins. This is thy only choice."