'pretty girl'- pretty is being used as an adjective, indicating being
attractive in a delicate way.
However, it can also be used informally and ironically in everyday speech to express annoyance or disgust : "So much for loyalty between friends. It is a pretty state of affairs when your own best friend turns you in to the Gestapo."
It can also be used informally to mean, 'to a moderately high degree' : he looked pretty fit for his age.
Does that help?