@Liquidxlax: Technically what she call her is "しっぽ顔" which if translated as is would be 'tail face'
But given that it's written in hiragana so it can be any kanji that fit, maybe Fol was trying to say 'shippoi'(しっぽい) as adjective, one variation of which is 死っぽい 'dying'
And guess how Chastille was in ch 19....
Just speculating tho, since I can't find raw of the novel to check if Fol ever give explanation for that.
@Liquidxlax, @Meridis@WhimsiCat: I interpreted "tail-face" as a weirdly phrased, intended-to-be-disparaging reference to Chastelle having a ponytail hairstyle.