Robert Underdunk Terwilliger, better known by his stage name Sideshow Bob
Lionel Hutz
Troy McLure
Patty Bouvier
Selma Bouvier-Terwilliger-Hutz-McClure-Stu-Simpson
A few years ago they implied she briefly remarried Sideshow Bob, which I thought was hilarious.
Groundskeeper Willie
Mrs. Krabappel
Moe Syslak
Waylon Smithers
Krusty the Klown
Lenny
Karl
Charlie
Otto Mann
Seymour Skinner
Superintendent Chalmber
Martin Prince
Milhouse (and Kirk and Louanne) Van Houten
Kearny
Dolph
Jimbo
Nelson
Eddie
Lou
Snake
Agnes Skinner
Reverend Lovejoy and Helen Lovejoy
Apu and Manjula Nahasapeemapetilan
[I don’t think I can name all eight kids… Geet, Uma, Sashi, Priya… eh, I’ll come back to it]
Gil Gunderson
Kang and Kodos
Dr. Marvin Monroe (does he still count as a regular?)
Dr. Nick
Dr. Hibbert
Dr. J. Loren Pryor
Blue-Haired Lawyer
I’ll post this now before all my entries are ninjaed.
Fat Tony
Legs
Louie
Jeff “Comic Book Guy” Albertson
Grampa Simpson
Jasper
Mr. Largo
Sherri and Terri
Louis, Wendell, and Richard
Mrs. Glick
(going way back) Mrs. Winfield
Database
Mayor Quimby
Professor Frink
Cletus and Brandine Spuckler and their kids… including but by no means limited to Q-Bert, Zoe, Chloe, Phil, Rumer, Scout, International Harvester, Incest, Stabbed in Jail, Mary, and their cousin, Dia-betty
Kent Brockman
Cy in the Sky (is that his name?)
Bumblebee Guy
Itchy & Scratchy (But not Poochy - he sucked.)
Booberella
Princess Kashmir
Squeaky-voiced Teenager
Sailor Guy
The Squeaky-Voiced Teen
Aristotle Amadopolous
Akira
Sanjay Nahasapeemapetilan
Scott Christian
I don’t think Mayor Quimby’s wife or mistress have ever been named, but they’re both regular characters