I figured the concept of a Chainsaw Ninja wasn't really original, but it's not too common, either. When I actually drew my character, he actually turned out to be a Soviet Chainsaw-wielding Ninja Santa wearing a Business Suit. I think that's... kind of original?
As for just about every other username, character name, etc. I am very strict about trying to make it as original as possible by taking random last names from different parts of the word and throwing them in a blender. My most used one, Eingzheider, is a success story compared to most of the creations I get, but I usually find a purpose for each and every name I come up with.