me: she’s not particularly good looking
  but has a certain
  sinewy quality
  and a mouth
  that could only be described as ‘pert’
 Allison: oh dear
  

pert/pərt/

Adjective:

  1. (of a girl or young woman) Sexually attractive because lively or saucy.
  2. (of a bodily feature or garment) Attractive because neat and jaunty.

Synonyms:

saucy - impertinent - cheeky - insolent - impudent