Keith Hernandez is a retired Major League Baseball player. Jerry and George met him in the locker room of their health club. He is a big fan of Jerry's comedy and he becomes awkwardly entangled with Elaine and Jerry. Kramer and Newman hate him, though, and they accuse him of spitting on them after a game (Newman recalls that it was June 14, 1987, Mets/Phillies, in which the Mets blew a ninth-inning lead and caused the Phillies to rally for the win). Jerry meticulously proves their theory is false, JFK style. They later find out that it was his teammate, relief pitcher Roger McDowell, who had spit on them instead. Kramer and Newman apologize to Hernandez, and they help him with his moving. ("The Boyfriend", two-part episode)

In "The Finale, Part 2," Keith Hernandez attends the trial of Jerry, Elaine, George, and Kramer in Latham County. A deleted scene before the trial had him telling Newman that he has been doing some announcing. He also tells Newman that he is staying at the Ramada during the course of the trial. Not being able to get a room anywhere, Newman suggested to Keith that he can have a cot put into his room only for Keith to turn down that suggestion. When Jerry, Elaine, George, and Kramer are brought into the courtroom, Kramer calls out to Keith who just nods back at him. While the jury was out deciding on the verdict, Keith was among the trial attendees seen at the pool house where plays a game of pool against Jacopo Peterman, Kenny Bania, and Mickey Abbott for money.

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