The first forum I ever joined was The Official Elder Scrolls Forum. I'm not very good at coming up with names and couldn't think of a good one to use, so I just used the name of my best character in Morrowind, Marlot the Conjuror. He was a mage specialising in conjuration and the only character I have beaten the main quest with, he also reached the top of the Telvanni guild within a very short time. I'm not sure how I came up with that name, but I liked how it sounded like a real-life magician who would perform at some cheap, back alley theatre.
I can't remember the next time I had to come up with a username but whenever it was I considering using Marlot the Conjuror but felt it was a little strange for general use so I shortened it to M_the_C. It has been fun hearing various people trying to pronounce the underscores in multiplayer games that support microphones. If I were ever to speak it I would simply say "em the sea".
When it came time to join Wurm Online I found it didn't support underscores so I shortened it again to MtheC, but Wurm then de-capitalised the C since your username is meant to be just that, a name. I think I actually prefer the way it turned out, Mthec.
In various multiplayer situations I've been further shortened down to just M, I'm perfectly happy with this since time can often be an issue. This has actually grown on me because it makes me feel like I may be M from James Bond.