On “Seinfeld,” Kramer’s rarely used first name is Cosmo. The show usually calls him only Kramer, making the first-name reveal one of the character’s more memorable pieces of background.
George learns the name from Kramer’s mother, Babs, in the season-six episode “The Switch.” The episode’s title refers to a scheme involving Jerry’s apartment and a temporary apartment swap, while the family revelation gives viewers the character’s full name: Cosmo Kramer.
Kramer was played by Michael Richards and was loosely based on comedian Kenny Kramer, Larry David’s former neighbor. The character originally had the name Kessler in the pilot, partly to avoid legal complications, before the show adopted “Kramer.”
Cosmo is the canonical first name, but the series also gives Kramer several comic aliases, including H. E. Pennypacker and Professor Van Nostrand. An unused episode concept reportedly considered “Conrad,” but that name never became part of the aired series.