Kramer is a semi-fictional character from the TV series Seinfeld. His full name is Cosmo Kramer and he is played by Michael Richards.
Like many of the characters in Seinfeld, Kramer has numerous running gags, probably having even more prominent ones than any other character.
Kramer's unique entrance
Kramer is most well known for his unique form of entering the apartment. It usually involves a quick burst onto the stage, opening the door and sliding in all in one motion.
Like many famous sitcom next door neighbors, Kramer is a world class mooch, he is constantly coming over to Jerry's place and "borrowing" food from him or using his apartment for his own needs, such as cooking up large meals or having girls over.
Mystery Friends and Apartment
Kramer is a big mystery in the series, his interactions with Jerry and his friends being the only part of his life we see. Because of this we only get vague references to the inside of his apartment (for example, we learn that it has chicken wire inside because it may or may not contain a chicken) and he is constantly referencing friends of his that we never see, such as Bob Sacamano (who according to Kramer has had rabies, works at a condom factory, and once stayed with him for a year and a half) and Lomez (an Orthodox Jew who Kramer purchased a hot tub from).
Source of Money?
Kramer never has a steady job, which constantly raises the question with Jerry "where does he get his money?" While it is a joke running throughout the show's nine seasons with never a clear answer, fans of the series actually can notice that he does indeed have some sort of income.
Kramer, as previously mentioned, mooches food off of Jerry which cuts down on his monetary needs.
Kramer once was an underwear model for Calvin Klein
He once wrote a coffee table book about coffee tables, which in the last season is bought for a movie.
Probably his most common source of income is his gambling wins. He is a compulsive gambler and even though the fact that he is trying to stop implies he isn't good at it, the few times we see him gambling he actually wins big, once turning a $600 bet into $18,000.
Real Life Inspiration
Kramer was based on Larry David's real life next door neighbor Kenny Kramer. According to Larry David, Kenny was always coming over to borrow food and having hair brained schemes. After the series became popular Kenny Kramer got a bus and started his Seinfeld Reality Tour, which later they put into the show by having the character Kramer sell his life stories to J Peterman and then starting a "Peterman Reality Bus Tour."