Sofia

While alive, they were Kaya's Ersatz. They use they/she pronouns and her gender is pretty much just Not A Girl.

A long time ago, Specter, a gang of semi-mortals, killed her and her whole village. Unfortunately, this happened in the middle of the Black Plague, so since Lyn and Connor were so busy her soul never got collected and properly brought to the Underworld. This resulted in her being caught in a state between life and death, so she is what is known as a semi-mortal: a dead person who can freely pass between different sectors of the Underworld as well as between the Underworld and Overworld. However, they can also die for good, without any form of afterlife or reincarnation.

They see this state as a curse, and so they dedicate their afterlife to hunting down and killing anyone involved in Specter AND the Shinigami, who she blames partially for her fate.