She’s a corpse reanimated and possessed by the Blood Devil (a “fiend” in the lore), and starts off with zero human sensibilities. She was basically feral.

She becomes more human over time. Starting with a cat that she initially intends to eat, but which she ends up so attached to that she tries to sacrifice the main character to the Bat Devil so it would give back her cat.

And that’s how she met Denji and entered the story.