These are characters from the animated TV series 
The Simpsons.
Warning: Wikipedia contains spoilers 
Characters are only listed once, normally under the first applicable subsection in the list; very minor characters are listed with a more regular chracter they are associated with. Subsections are ordered by size and ongoing relevance to the show.
The Simpson family members
-  Homer J. Simpson, father
-  Marge Simpson, mother
-  Bart Simpson, son
-  Lisa Simpson, daughter
-  Maggie Simpson, baby daughter
-  Abraham J. Simpson, Grandpa
-  Mona Simpson, Homer's mother
-  Selma Bouvier Terwilliger Hutz McClure, one of Marge's twin sisters
-  Patty Bouvier, the other of Marge's twin sisters
-  Jackie Bouvier, Marge's mother
-  Marge's father, seen only in flashbacks
-  Herb Powell, Homer's half brother
Other families
-  The Flanders family 
-  Ned Flanders, father, owner of Leftorium
-  Maude Flanders, mother (deceased)
-  Rod Flanders, son
-  Todd Flanders, son
-  Grandma Flanders (senile and decrepit several years earlier; probably dead?)
-  ? (Ned is known to have at least one sister)
 
-  The van Houten family 
-  Kirk Van Houten, cracker factory employee
-  Luann (changed her last name back?), his ex-wife
-  Milhouse Van Houten, their son, Bart's best friend.
-  Milhouse's grandmother
 
-  The Nahasapeemapetilon family
-  Apu Nahasapeemapetilon, a convenience store owner
-  Manjula, his wife
-  Their octuplets (by stage name):
-  Sanjay, Apu's brother
-  Pahusacheta, Sanjay's daughter
 
-  The Wiggum family
-  Clancy, the incompetent police chief
-  Sarah, Clancy's wife and Ralph's Mom
-  Ralph, the mentally challenged son
 
 Institutions 
-  Springfield Elementary School
-  Seymour Skinner, principal, who lives with his mother Agnes.
-  Superintendent Chalmers 
- Teachers
-  Miss Elizabeth Hoover, second-grade teacher
-  Edna Krabappel, teacher.
-  Dewey Largo[?], music teacher
-  Mr. Glascock[?], teacher, now retired from the stress of children making fun of his name
-  Mrs. Cummerdale[?], gym teacher
-  Mr. Bergstrom, substituted for Miss Hoover, (voiced by Dustin Hoffman)
 
-  Otto Mann, bus driver
-  Groundskeeper Willy
-  Lunchlady Doris/Nurse Doris
-  Students
-  Jimbo Jones
-  Kearney
-  Dolph
-  Nelson Muntz, school bully known for his scornful laugh
-  Martin Prince an overachieving nerd
-  Sherry and Terry purple-haired twins whose dad has a high-up position at the power plant.
-  Üter, German exchange student
-  Wendell, a nervous student who vomits frequently
-  Louis[?]
-  Nerd based on Saturday Night Live's Pat
-  Oliver[?], a nerd encountered briefly when Bart was brought to their secret hideout
-  Ralph Wiggum Lisa's below-average classmate
-  Allison Taylor, Lisa's fellow-overachieving classmate
 
 
-  Moe's Tavern 
-  Moe Syzlak, owner and proprieter
-  Barney Gumble, boozehound
-  Hugh Jass
-  Joey Jo-Jo Junior Shabadoo
-  Aerosmith, briefly the house band
-  Waitress Moe hired during the "Flaming Moe's" period
-  Woman Moe dated briefly
 
-  Government, Police, Law
-  "Diamond" Joe Quimby, Mayor
-  Chief Clancy Wiggum, Chief of Springfield Police
-  Lou, police officer
-  Eddie, police officer
-  Snake, a recidivist crook with a well-educated accent
-  Judge Synder[?], veteran judge in Springfield known for lenient punishment
-  Constance Harm[?], newer judge known for vindictive punishment, a parody of Judge Judy[?]
-  Lionel Hutz, incompetent lawyer
-  Burns's Lawyer[?], a pasty faced lawyer who always opposes the Simpsons in court
- Related:
-  Joe Quimby's wife and numerous mistresses
-  Quimby's nephew (brother?) Clovis
 
 
Local television and movie personalities
-  Krusty the Klown, real name Herschel Krustovsky, entertainer
-  Rabbi Hyman Krustovsky, Jewish Rabbi and Krusty's formerly estranged father
-  Sophie, Krusty's daughter (voiced by Drew Barrymore)
-  Lois Pennycandy, his assistant
-  Mr. Peepers, his monkey
 
-  Sideshow Bob, Robert Underdunk Terwilliger, the attempted-homicidal former sidekick of Krusty
-  Sideshow Mel, Bob's humble but eloquent replacement
-  Kent Brockman, local anchorman for KBBL-TV (Channel 6)
-  Arnie Pie[?], Channel 6 traffic reporter
-  Bill and Marty[?], local radio personalities on KBBL-FM
-  Troy McClure, a washed-up movie star
-  Rainer Wolfcastle aka McBain, a famous Hollywood action hero
- Rainer Wolfcastle's daughter
- Rainer Wolfcastle's wife (?), Maria
 
-  Bumblebee Man, a Mexican TV show host (Channel Ocho) always dressed in a bumblebee suit
-  Roger Meyers, Jr.[?], chairman of "Itchy and Scratchy International" who briefly hired Grampa Simpson as a writer
- Chester J. Lampwick, creator of cartoon violence
Musicians
Other recurring characters
-  Artie Ziff[?], Marge's prom date in high school, now a nerdy software millionaire.
-  Baby Gerald[?], Maggie's enemy, has unibrow
-  Captain Horatio McAllister[?], a seafood resturaunt operator, now known simply as 'the Sea Captain'.
-  Cletus Delroy, a slack-jawed yokel
-  Comic Book Guy, the local comic book shop owner
-  Disco Stu a crazy Disco person
-  Dr. Julius Hibbert, the Simpson family physician
-  Bernice Hibbert, his wife
 
-  Dr. Marvin Monroe, a psychiatrist (deceased)
-  Dr. Nick Riviera, impersonates a doctor
-  Drederick Tatum, professional boxer, parody of Mike Tyson.
-  Duffman, spokesman for Duff Beer
-  Fat Tony, a mafia leader
-  Joey, one of Fat Tony's mobsters
-  Legs, another mobster with a short ponytail
-  Louie, mobster
 
-  Gil, a perpetual loser
-  Hans Moleman, a short, elderly man with very bad luck
-  Jack Larson[?], slick spokesman for Laramie cigarettes
-  Jasper, Grandpa Simpson's best friend
-  Jebediah Springfield, the historical founder of Springfield.
-  Luigi, proprietor of the local Italian restaurant.
-  Professor John Frink, brilliant scientists and local Springfield professor
-  Reverend Timothy Lovejoy the local priest
-  Helen Lovejoy, his wife
-  Jessica Lovejoy, his daughter, a troublemaker who briefly went out with Bart
 
-  Herman, the owner of a military antiques store
-  Squeaky Voiced Teen, a teenager trapped in a long list of dead-end jobs
Other one-time characters
Characters who are fictional even within the context of The Simpsons
Characters that have died during the course of the series
- Maude Flanders
- Bleeding Gums Murphy
- Dr. Marvin Monroe
- Frank Grimes Sr.
- Snowball I
- Hans Moleman (faces some sort of new death every week.)
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