Having recently done a list of Italian characters, I thought I'd try a list of Jewish characters:
Definitely
Jerry Seinfeld Helen and Morty Seinfeld, Jerry's parents Leo, Jerry's Uncle and Helen's brother Stella, Jerry's Aunt (The Pen) Nana, Jerry's Grandmother and Helen and Leo's mother Mr. Lippman, Elaine's boss at Pendant Publishing Adam Lippman, Mr. Lippman's son (The Serenity Now) Rabbi Glickman from Elaine's apartment building (she calls him Rabbi Kirschbaum in The Postponement) Tim Whatley, Jerry/Kramer/Elaine's dentist Mac, Jerry's Uncle (The Stakeout) Artie and Carol Levine, Jerry's cousins (The Stakeout) Myra and Stan Flick, and their baby Steven (The Bris) The Mohel who performed the bris on baby Steven Rachel Goldstein, Jerry's girlfriend (The Raincoats, The Hamptons, The Opposite) Mr. Goldstein, Rachel's father (The Raincoats) Fred Savage (The Trip) Mel Torme (The Jimmy) Bette Midler (The Understudy) Eddie Sherman, the fatigue-wearing psycho at J. Peterman (The Fatigues) Geraldo Rivera (The Finale)
Jewish Characters frequently mentioned but never appeared
Lomez, Kramer's friend Jeffrey, Jerry's cousin and Uncle Leo's son
Possible (but can't confirm)
Estelle Costanza, George's mother (plays Mah Jongg, won't ride in a German car, house smells of kasha (Jewish cuisine), ...) George Costanza (if his mother is Jewish, then he is Jewish, seems to know Jewish men like to date non-Jewish women due to their "shiksappeal") Estelle Costanza's Mah Jongg girlfriends (The Handicap Spot) (Mah Jongg is a game that as a stereotype is played by elderly Jewish women) Joel Horneck, Jerry's childhood friend (Male Unbonding) (Jerry would presumably have Jewish childhood friends, Jewish sounding name, Joel is a Hebrew name) Manya and Isaac, (The Pony Remark) (Manya's Yiddish accent, Isaac is a Hebrew name) Levitan, George's real estate boss (The Revenge) (Jewish sounding surname) Tabachnick the bootleg cable installer (The Baby Shower) (Russian Jewish sounding surname, many Russian Jews live in NYC) Mr Cohen the Chinese Restaurant regular (The Chinese Restaurant) (Cohen is a Hebrew surname) Joe Bookman, Library Cop (The Library Cop) (Jewish sounding name) Sherry Becker, Jerry's high school classmate (The Library Cop) (Jewish sounding name) Jack and Doris Klompus, Jerry's parents' neighbors (Jerry's parents presumably have Jewish friends/acquaintances, Jewish sounding names) Evelyn, Jerry's parents' neighbor (ditto) Old man/Polar Bear Club member at the Intervention (The Pez Dispenser) (his Yiddish accent/use of words, acknowledged in DVD commentary) Mrs. Lenore Sokol, George's Unemployment caseworker, and her daughta' Carrie (The Boyfriend) (Jewish sounding name) Doctor Dembrow (The Wallet) (Jewish sounding surname) Naomi, Jerry's funky-giggling girlfriend (The Watch, The Bubble Boy) (Naomi is a Hebrew name) Shelly, George's cousin (The Contest) (if his mother is Jewish) Mr & Mrs Harold Reichman, George's friend's parents (The Opera) (Jewish sounding surname) Sharon Leonard, NYU student (The Outing) (Jewish sounding name, Sharon is a Hebrew name) Sid Fields, old man with record collection (The Old Man) (Jewish sounding name, named for the real Sid Fields, Jewish actor of Abbott and Costello fame) Dr Siegel (The Junior Mints) (Jewish sounding surname) Sal Bass, Salman Rushdie lookalike (The Implant) (Jewish sounding surname, has been to the Middle East) Barry Profit, Jerry/Kramer/Newman's accountant (The Sniffing Accountant) (Jewish sounding name) Jake Jarmel, Elaine's boyfriend and author (Jewish sounding name, Jacob is a Hebrew name) Arthur Pensky, George's client of the Pensky file (The Barber) (Jewish sounding name) Joel Rifkin, Elaine's boyfriend (The Masseuse) (Jewish sounding name, Joel is a Hebrew name) Stu Chermak, NBC employee (The Pitch, The Ticket, The Virgin, The Pilot, The Finale) (Jewish sounding name) Rita Kierson, NBC president (The Virgin, The Pilot) (Jewish sounding name) Sylvia, the furniture repair store employee (The Cigar Store Indian) (common Jewish female name, recognized the smell of kasha from George's parents' house) Aaron, Elaine's boyfriend, the close talker (The Raincoats) (Aaron is a Hebrew name) Jocelyn Landis of Doubleday (The Chaperone, The Switch) (Jewish sounding surname) Ronnie Kaye, the prop comic (The Fire) (Jewish sounding name) Aronson, the Poland Spring/Morgan merger businessman (The Gymnast) (Jewish sounding surname) Gary Fogel, Jerry/George's lying about cancer friend, (The Scofflaw) (Jewish sounding name) Mr. Mandel, interviewing Elaine at Viking Press (The Doodle) (Jewish sounding surname) Sid Farkus, Frank Costanza's bra salesman friend, (The Sniffing Accountant, The Dinner Party) (Jewish sounding name) Dr. Howard Cooperman, aka The Assman, Frank Costanza's Proctologist (The Fusilli Jerry) (Jewish sounding name) Alec Berg, Jerry's friend (The Face Painter) (Jewish sounding name, and great "John Houseman" name) Fred Stoller, Elaine's forgetful date, (The Secret Code) (Jewish sounding name) Mrs. Mabel Choate, Jerry's parents' neighbor and marble rye victim (The Rye) (see above about Jerry's parents' neighbors, shops at Schnitzer's, a Jewish delicatessen) Herb, Ralph, "Bldg A", "Bldg B", and "Bldg C", Jerry's parents' neighbors (The Cadillac) (see above about Jerry's parents' neighbors) Bob Grossberg, hard-of-hearing Peterman employee (The Friars Club) (Jewish sounding name) Ethan the gay Wigmaster (The Wig Master) (Ethan is a Hebrew name) Jeannie Steinman, Jerry's fiancee (The Invitations, The Foundation) (Jewish sounding surname) Ned Isakoff, Elaine's communist boyfriend (The Race) (Jewish sounding name) Dr. Stern and Dr. Resnick, Elaine and Uncle Leo's Doctors (The Package) (Jewish sounding surnames) Cynthia Pearlman, the mentor and Banya's girlfriend (The Fatigues) (Jewish sounding surname, presumably took Banya to Kramer's Jewish singles event) Ben the intern, Elaine's boyfriend (The Abstinence) (Benjamin is a Hebrew name) Mr. Shellbach, Kramer's living will attorney (The Comeback) (Jewish sounding name) Ruthie Cohen, Monk's Cashier (The Gum, The Foundation, plus cameos in many others) (Ruth and Cohen are Hebrew names) Emily, Kramer's girlfriend with the jimmy legs (The Money) (she took Kramer to Schnitzer's, a Jewish deli, and her friends are the Feinermans, a Jewish sounding surname) Steven Koren, disputed Susan Ross scholarship winner and member of the Van Buren Boys (The Van Buren Boys) (Jewish sounding name) Izzy Mandelbaum, neighbor of Jerry's parents (The English Patient, The Blood) (see above about Jerry's parents' neighbors, Jewish sounding name) Izzy's father (The English Patient) and son (The English Patient, The Blood) Shlomo, Frogger truck driver (The Frogger) (Shlomo is a Hebrew name) Sid Luckman, Jerry's parents' neighbor in the exercise room at Del Boca Vista (The English Patient) (see above about Jerry's parents' neighbors, Jewish sounding name) "Fragile" Frankie Merman, Jerry's childhood friend (The Junk Mail) (see above about Jerry's childhood friends like Joel Horneck, Jewish sounding surname) Lou Filerman, Elaine's sidler colleague at J. Peterman (The Merv Griffin Show) (Jewish sounding name) Gwen, Jerry's two-faced girlfriend (The Strike) (met Jerry at Whatley's Hanukkah Party) Mr. Elinoff, New Yorker magazine (The Cartoon) (Jewish sounding surname) The Old Men who couldn't use tip calculators (The Wizard) (Jerry's parents' neighbors) Irwin Lubeck, Peterman's pastry appraiser (The Frogger) (Jewish sounding name) Bobby, grandson of the late Gammy Kranz (The Maid) (Gammy has Jewish sounding surname)
More of a Stretch
Meryl, Jerry's fake wife (The Wife) (female name used by Jews and non-Jews) Carol (You Gotta See the Bay-Bee), Michael, and their Bay-Bees, Jerry/Kramer's former neighbors (The Boyfriend (2), The Hamptons, The Soul Mate, The English Patient) (Hamptons/ugly baby Adam is a Hebrew name, though Michael being the Jewish son of a lobsterman is a bit of a stretch) Mel Sanger, the bubble boy's father (The Bubble Boy) (name sounds Jewish, but they live in a presumably non-Jewish area upstate) Mrs. Sanger and Donald the Bubble Boy (if Mel is Jewish) Dr. Bakersoll, Estelle's eye surgeon (The Fusilli Jerry) (made-up Jewish sounding surname) Donna Chang(stein), Jerry's girlfriend (The Chinese Woman) (made-up Jewish sounding surname) Dr. Beth Lookner, Elaine/Jerry's friend (The Wait Out, The Yada Yada) (obvious Jewish sounding name, but she made an anti-semitic remark in The Yada Yada) David Lookner, Beth Lookner's first husband (The Wait Out) (Jewish sounding name) Arnie, Beth Lookner's second husband (The Yada Yada)