In English this functions as an adjective and a noun but in Japanese Bishounen and Bishoujo are only nouns.
Originally from the Japanese word: Bishounen
Bi, sho, nen.
Each character meaning: Beautiful small year; respectively, roughly meaning "pretty
boy"
Pretty girl is somewhat different:
Bi sho jou.
Again each character meaning: Beautiful small
woman, respectively. Roughly meaning "pretty girl"