I've been using this name everywhere since I was 15 and made my very first Runescape account. >.>
At the time, I was a massive Greek mythology buff and very fond of swimming. One of my favorite things to do was dive underwater, swim the entire length of the park pool, breast-stroking along the bottom and then suddenly pop up beside a friend or family member, scaring them half to death.
I named myself after the Nereids, sea-nymphs, daughters of Poseidon - scary creatures, given the chance to be. As for the 13, that's always been an important number to me - it's the date of my birth, one of my mother's childhood addresses, the day my parents were married, purportedly an unlucky number (I have had Friday the 13th birthdays >.<), but it's brought nothing but luck to me.