Answer by taking a quote from the following article:

Cosmo Kramer, usually referred to as simply "Kramer", is a fictional character on the American television sitcom Seinfeld (1989-1998), played by Michael Richards. The character is loosely based on comedian Kenny Kramer, Larry David's ex-neighbor across the hall. Kramer is the friend and neighbor of main character Jerry, residing in Apartment 5B, and is friends with George and Elaine. Of the series' four central characters, only Kramer has no visible means of support; what few jobs he holds seem to be nothing more than larks.

In "The Trip", Kramer admits that a man in a park exposed himself to him when he was a young boy. In "The Big Salad" Kramer reveals to Jerry that he grew up in a strict household where he had to be in bed every night by 9:00PM. In "The Letter", Kramer tells two art patrons that he ran away from home at age seventeen and stowed away aboard a steamer bound for Sweden.  Kramer never completed high school; however, it is made clear in "The Barber" that Kramer has a GED.  Kramer was estranged for a long period from his mom, Babs Kramer, who works as a restroom matron at an upscale restaurant. Unlike George and Jerry, Kramer's character does not have a well-developed network of family members shown in the sitcom. He is the only main character on the show whose dad never makes an appearance; however, in "The Chinese Woman", Kramer mentions that he is the last male member of his family, implying that his father had died. He also mentions in "The Lip Reader" that he has or had a deaf cousin, from whom he learned fluent American Sign Language, but when Kramer tries to communicate in ASL, he speaks complete gibberish and cannot correctly translate the ASL he sees others using. He also apparently has no biological children, although he adopted mile 114 of the Arthur Burkhardt Expressway in "The Pothole".  During an opening discussion, Kramer reveals to Jerry that in 1979 he was struck on the head by a falling air conditioner while walking on the sidewalk. Jerry asks if that was when Kramer lived in Greenwich Village, to which Kramer replies that he cannot remember. This is discussed in the beginning of "The Little Kicks".  In "The Strong Box", it is revealed that Kramer spent a brief time in the Army, although info about this time is "classified". In episode three of season one he says he lived in Los Angeles for three months.

How was his relation with his mom?
Kramer was estranged for a long period from his mom,