My username was actually made up by my uncle Bret. I was about 8 or younger, and I was playing Guitar Hero 1 (believe it or not, the first one existed. It's just under-appreciated). So I started my file on there, and I was having trouble coming up with a name. My uncle just told me to use my own name, but with a 3 instead of an E. I know it's kinda cheesy, but it stuck.
As for my Xbox LIVE Gamertag, well, I don't have my own, so I use my dad's.