My username is from way back when I was 10 and used 3D Game Maker to make a game (not Gamemaker's 3D - I mean the much simpler tool actually called 3D Game Maker). It prompted me for an author name and I came up with a fictitious company called Docopop and signed it that. Later when I made an account on what is now the YoYo games sandbox I called myself Docopoper as in "a member of Docopop". At this point however I don't really consider Docopop a thing and just consider myself Docopoper independently. The name Docopop came from my ten year old self thinking of Coco Pops and just altering the name a bit. In fact it's actually supposed to be pronounced similarly to Coco pops for all you people applying different stresses to parts of my name (though at this point so many people do it that I don't mind).
Avatar:
My avatar is of Tick Tock Clock since it is objectively and in every way possible the best level ever made in any game. I took the actual image of it directly from Tick Tock Clock's wiki page.
Signature:
Spoilered in case you would rather have your own interpretation of my Signature.