I've gone through a lot of usernames over the years, the first was Mystii, that was the name of my dog. Then I went through a LotR phase and my elvish name - Alasse - came out of it, then Mirwyn after I stopped being stupid. Then I hit a giant anime phase and Mikiru was used for a while, then Sanke (the name of one of my RP characters). My immediately previous username, Rawhn, was given to me, randomly generated by a neopets password reset code generator. I liked it, so I used it. And mekashiyu, my current one...I jacked it. There was a type random letters thread on GaiaOnline (from back when I was into that stuff) where someone's button-mashing induced the characters "mekashiyu." I thought it was cool so I took the name and ran with it. I haven't been caught yet and it's unique enough to never be used anywhere else, so yay. >.>
Oh and also my backup accounts all have the username hasarx, which is my nobody name. I went through a Kingdom Hearts phase too, yeah. I'm really impressionable, it seems.