Arielle does smile from time to time, but normally they only smile around their best friend to make him feel that he is appreciated.
Arielle is apathetic, though, so smiling isn't really something they do genuinely. It's an expression they learned how to show when times need for it, but other than that they don't smile. The village knows them to have an "icy appearance" and that they seem distant, but Arielle always seems like a nice person in general so they don't give it too much of a thought.
Also there's the saying you could smile with your eyes? Yeah. Arielle's eyes always seems distant.
I wouldn't say dead, or lifeless, but I'd say more like emotionless, which truly reflects their internal feelings.
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Lily generally doesn't smile either.
She stopped smiling for a very long time, after her brother's murder, and only started to warm up after the case was solved.