I'll explain both my usernames so far.
» Cowrie - I've explained this in more detail in the past, but basically: had a character, named that character "Cowrie" after remembering a question I had in some big book of questions and answers as a kid. Took that name when I first joined PC, since I suck at coming up with names and wanted something unique. For the record, a cowrie is a marine snail.
(Though if I'd known to what extent I'd be getting certain jokes by now, I might have taken the time to choose another name, hurr.)
» Sylphiel - taken from one of my recurring obsessions, the anime series Slayers; it's the name of one of the characters. She's not necessarily my favorite character, but she did have my favorite name from there. I think I've loved that name (or at least, that particular transliteration of it) ever since I first saw that series.
...So when I finally got the courage to change my username, it was one of the choices I had (and the choice that won, obviously).