My original username was XBassXUmbraX on the very first online community I joined, which was gamefaqs (and still is, since you can't change your name there.) It was 2003 and I was 13, so I thought X's in your username were all the rage back then. Bass of course comes from Mega Man's rival, as Mega Man is probably my favorite video game series of all time. The Bass I am referring to though was from the Mega Man Battle Network series, who was a difficult rogue secret boss in the first few games of that series, and honestly a lot more badass than the one from the original Mega Man series. The Umbra was just filler that seemed to sound cool I guess.
About a year later I joined several other sites where just plain old "Bass" was available, so it is a name I stuck with ever since. You would be surprised, but almost every site I go to, I am asked "Is the Bass in your name the guitar or the fish?" Of course, you should all know by now that the answer I would reply with is "Neither, it is the video game character." Nevertheless, this has made my name the butt of many jokes even today, considering how often I am needlessly highlighted on IRC these days. Anyway, just about every username I have used on any online community has been a derivative of Bass (that includes my IRC nick, Forte, which is just the Japanese name for Bass) as well as BassDX on sites where Bass wasn't available.