Apr 7
As you know by now, Ceylon doesn't wear a mask; as a result, she has sensory processing issues and can get overstimulated.
When the world becomes too loud or the information too dense, her composure fractures. Her shell hair begins to bleed into a void like shadow, manifesting her internal overwhelm as a physical corruption.
However refusing the comfort of a mask allows her to read the air, the soul, and the history of a room with precision.
And yes, that's why she wears a collar.
In a similar way, I imagine that Quirrell covers his antennae because they are more sensitive.