She was played by Brenda Strong.
As detailed in Season Seven's "The Caddy," Sue Ellen and Elaine did grow up together. According to Elaine, Sue Ellen stole her boyfriend during a party back in 1980; while wearing a provocative outfit. Elaine nicknamed her "The Braless Wonder," due to the fact that she never wears a bra. The episode also reveals that Sue Ellen is the heiress to the Oh Henry! candy fortune.
Sue Ellen first appeared in "The Caddy" in Season Seven and she runs into Elaine and tells her that her birthday is coming up. As a gag gift, Elaine gives Sue Ellen a bra as a gift, which she later shows Elaine at her workplace. However, Sue Ellen is wearing the bra as a top, under a black blazer, much to Elaine's bewilderment. Jerry and Kramer later spot Sue Ellen walking down the street in her bra, and the distraction leads to the two getting into an accident in their car. Because Kramer's golf game is affected by the injuries suffered in the crash, Elaine cooks up the idea of suing Sue Ellen. Jerry begins falling for Sue Ellen when they meet again at his apartment, and Sue Ellen counted on Jerry to help her in the trial because of that. The case is thrown out when, under the advisement of Kramer's caddy, Sue Ellen tries on the bra, and it doesn't fit.
Sue Ellen plagued Elaine again later in the season in "The Bottle Deposit" and Season Eight's "The Abstinence." In the former, Sue Ellen gets in a bidding war with Elaine over golf clubs owned by former President John F. Kennedy; clubs that Elaine's boss, J. Peterman wants. In "The Abstinence," Elaine shows off her boyfriend to Sue Ellen and tells him that he's a doctor, but he's actually in school to become a doctor. It did not help Elaine's case when a civilian actually aided a choking victim and her boyfriend couldn't, which had Sue Ellen smirking in arrogant, villainous pleasure.
Sue Ellen's last appearance was in Season Nine's "The Betrayal," the famous backwards episode. She was getting married to her fiancee, Pinter, in his native India, and Elaine (with Jerry and George) flew there for spite; thinking that she wasn't invited. However, Sue Ellen was very happy to see Elaine; so much so that she wanted her to be not only her maid of honor, but her best friend as well, which Elaine happily accepted. However, Elaine revealed to Jerry that she slept with Pinter, who she knew under his Anglicized name, Peter, before he was engaged to Sue Ellen, and when it loudly comes out during the wedding, the friendship comes to an abrupt end. In a fit of rage, Sue Ellen actually ripped out Elaine's nose piercing.