When playing Smash with my siblings, we'd always have nicknames set when we played, just for fun n all that, and I took "Calamity" as mine, taking the name from the web series "No Evil". Calamity was my favorite character and design, and I also just liked how Calamity sounded. From there... I'm not actually sure how it shifted into MandatoryCatalyst? I needed something other than "Calamity" when setting up my Playstation account, and I figured MandatoryCatalyst sounded pretty cool. I had a different online handle at the time (CaptainAnabelle, and prior to that, PopcornSnuffler), but I like using multiple handles for anything that's not what I'd consider essentially my core few accounts I use (Steam/Discord/Flight Rising/ect)
The funny thing is is that I actually later switched off of MandatoryCatalyst, because it's too many characters for most websites, specifically I wanted it to be my Flight Rising username. Since I couldn't be MandatoryCatalyst, I switched to RetroCatalyst instead. However... since this is a forum where usernames aren't as strict as they are in other places, I figured, hey, might as well go with the handle I'd ORIGINALLY wanted for my online presence!
For my Splashtag, I just went with "Catalyst". On sites where I use "RetroCatalyst" people tend to shorten it to "Retro", but hey, not only do I somewhat prefer Catalyst for my handle over Retro, but "Catalyst" is what I view as the defining part of my username, so it fit better!
...technically my name is Jay but hey, here especially I want people to recognize me if they see me in matches, so Catalyst as a name it is!