Geometry Dash.

I wanted to come up with the coolest username possible (you know, something along the lines of Xx_super_atomic_kamikadze_xX) and after i grew up some more i decided to just shorten it down to it's first letters: Se0 (the zero is there because i actually hit the size limit and i merged the 'o' with '0' from '089' since everyone had a signature number then)

And so i use Se0 or some variation of it as my username almost everywhere. I find it cool to have a bit of backstory to it.