Nereid is fearured in the collectible card, Nereid the Water Faerie , and a TCG card by the same name. It also seems the default avatar, Water Faerie, is a part of, or based off the collectible card, Nereid the Water Faerie.
Her descriprion on her collectible card is,
"Nereid is an elusive faerie who prefers to remain hidden. If you go sailing off the cost of Mystery Island you can sometimes hear her singing at night."
Her description on her TCG card is,
"Three hundred years under the water,
and not a wrinkle in sight!
Nereid the Water Faerie TCG card
The Water Faerie default avatar seems to be Nereid