- "I can't believe you're moving out. (Grabs Kramer) Kramer, is this true? Is it really happening? It's ... it's like a dream."
- ―Estelle to Kramer
Estelle Costanza is the stereotypical overbearing, obnoxious sitcom mother. Estelle spends virtually all her time screeching at Frank and George. She and Frank have a love-hate relationship: they seemingly can't stand each other, and yet stay together out of habit (or because no one else can stand them). They were briefly separated, however. They get back together in "The Rye" for unknown reasons, but it is clear that their relationship hasn't improved in the slightest.
George moves back in with his mother and father when he falls on hard times. When he is offered a job as a hand model he thinks that he will soon be out, but after Leslie pushes him into an iron, he loses his gig. ("The Puffy Shirt"). George wants to move out of their house because of his father and his strict rules, and eventually does so when he manages to land a job at the New York Yankees in "The Opposite".
George lives with Frank and Estelle until well into his adult life. He is often humiliated by her very existence, and largely blames her for his being a neurotic failure. He is still emotionally tied to her, however. During his parents' separation, he frets constantly over the collapse of his family — not because he loves or even likes them, but because he can't stand the thought of his parents being "out there" in the dating world with him. He also finds the idea of making two visits on holidays to be entirely distasteful.
Estelle maintains cool, distant relations with Jerry Seinfeld's parents, Morty and Helen, because she thinks they are snobs. The Seinfelds in turn avoid the Costanzas, not out of superiority, but because they (like most of the people who know the Costanzas) can't stand them. She has a flirtatious friendship with George's friend Kramer, however, which is the source of much friction between her and Frank.
In fact, Frank and Estelle sleep in separate beds (at least some of the time, although George did have sex in their bed in "The Cigar Store Indian"), not because of marital problems, but because Estelle has "the jimmy-arms," according to Frank ("The Money").
Although Estelle's ethnicity was never mentioned on the show, both Larry David and Jason Alexander considered her to be Jewish. David referred to George as "half-Jewish" in an interview , and Alexander stated that he had no fixed ideas about George's ethnicity until the casting of Harris as his mother, and said that "she can't be anything but Jewish."  In "The Strike," Frank Costanza refers to the genesis of Festivus as springing from an occasion in which he was buying a Christmas gift for George, showing that Christmas was celebrated in the household, likely by Frank who is Italian. In another episode, Estelle refuses to allow Frank to purchase a Mercedes because she "won't ride in a German car", which is a position taken by some Jewish-Americans uncomfortable with the auto maker's link to Nazi Germany.
Estelle has thwarted George's chances at dates many times (example being in "The Cigar Store Indian"). She believes almost everything that is important to George and her husband is stupid.
In "The Finale, Part 2," Estelle and Frank attend the trial of Jerry, Elaine, George, and Kramer in Latham County. While the jury was making a verdict, Estelle visits Judge Arthur Vandelay where she tries to get him to not impose a heavy sentence on George by doing something for him. When Judge Vandelay quotes "What do you mean," Estelle quotes "you know." When the four are found guilty, Estelle faints upon hearing this. Frank later tries to wake her up so that they can beat the traffic back to New York.
A blog dedicated to the legality of the issues that arise in Seinfeld episodes, Seinfeld Law, has analyzed many of the classic Estelle Costanza moments as they might have played out in a court of law.