Willow's brain dump.

Their body is a crude imitation of the human body, with their second/current incarnation being especially unstable. This causes a lot of health issues.

They have an "outer shell," so to say. Too much harm to their body will shatter the outer shell and send their original form back to Teyvat's leylines.

Lune's first incarnation had strong internal organs but a weak outer shell. For their second incarnation, creating their internal organs was rushed in favor of creating a stronger outer shell.

They're technically intersex, but everyone is under the impression that they're afab.

Adding onto the previous fact- Lune's reproductive organs don't work as intended, making them infertile.

Lune gets tired easily after any sort of physical activity. Even a quick jog can make them feel weak. Due to this, people often consider Lune to be clumsy during their mailman duties.

Even when eating full meals consistently, they often border on being underweight.