Nymphs are Others that were created by Dionysus and are partially divine. They remain under the god's care or that of his cultists. They tend to be playful and feminine and are roughly the female equivalent to Satyrs, and both are noted to be kind to "innocents".
Because they embody the ancient ideal of womanhood and female sexuality, they tend to be rather young in appearance. Sandra noted that, despite most of the ones she'd note predating the bible, they didn't look legal age as the ideal beauty of 200 BC was younger than she liked.