Some of my friends and I were huge fans of Dragon Ball Z back in the day. We would watch/talk/play anything related to the series; it was pretty nerdy. Fast forward a bit to the release date of Budokai Tenkaichi 3, and with us being devoted fans, we bought it.
I would always play as Captain Ginyu due to his awesome set of special attacks. Most notably, his ability to switch bodies with the other player being the most useful. It could easily turn the tide of almost any fight and is even more hilarious when you win like so.
Around the end of middle school I had more free time and freedoms to do productive things, like browsing the internet. I needed a username, and Captain Ginyu seemed to work ok. I've stuck with it for a few years now, but I prefer just Ginyu nowadays.