My friends and I decided to join the (at the time) most popular forum for teens in my country. Our names and nicknames were already taken and we didn't want to use numbers, so we helped each other come up with something based on our interests. You can probably guess what mine were

After that I used it
everywhere for many years, so it was an obvious choice when I joined CS back in 2008.
Funny enough CS is now the only place I use it. It started with having to pick something different elsewhere since people got to it first, but between websites/games/social media closing down, me losing interest or forgetting passwords, and just plain not being as into anime anymore... yeah. I've considered changing it here too, but it's a bit nostalgic at this point so I'm fine with it.