 On the very first episode, "Good News, Bad News", Kramers name is  actually Kessler. This is referenced during the last season of the  show. On the backwards episode in Season 9, "The Betrayal", it goes  all the way back to when Jerry first moves into his apartment. When  introducing himself to Kramer, Jerry says "I saw your name on the  board downstairs. Kessler, right?", to which Michael Richards  responds "Actually, its Kramer."