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"