I use a lot of different usernames. It all depends on what game or forum we're talking about. This particular name, though...
Well, Lemmings Universe was either the first or one of the first forums I ever joined, and I just decided to sign up as one of my characters -- Lem'ka, who exists in the Lemmings-based world I thought up many years ago. And so I've been Lem'ka in Lemmings-related things ever since.
When we switched to this forum, though, I had to change it slightly. It won't accept apostrophes in the username, so I just decided to change it to Lemika here. It's pronounced more like that, anyhow -- as opposed to Lemka, or Lemaka, or Lemeka... well, you get the picture.
It's actually still Lem'ka, though. It just doesn't look like it. XD
Let's see... the only other two you're likely to run into would be Damager (Unreal Tournament 2004) and Erochor (Counterstrike). Damager was thought up in a moment of disgust with my last username there (Dakatha, a relic from my days of playing Populous: The Beginning constantly. There I was Dakatha, or Dakatha_TDM). Erochor is the name of one of my D&D characters, a psychotic Dread Guard.
So. There you have it. Oh, and if none of those (or the other few I have lying around) work, I usually just call myself Enerjak. That usually only happens for multiplayer (as opposed to online) games, however.