The main boy characters. From left to right: Sid, Gerald, Stinky, Arnold, Harold, Eugene, Brainy
The main girl characters. From left to right: Sheena, Rhonda, Helga, Phoebe and Nadine
Characters
Arnold and his family
- Arnold, the calm, idealistic and benevolent central protagonist.
- Grandpa Phil, Arnold's cheerful and wisecracking grandfather.
- Grandma Gertie, Arnold's eccentric and boisterous grandmother.
- Mitzi, Grandpa Phil's haughty and arrogant twin sister.
- Arnie, Arnold's monotonous cousin.
- Miles, Arnold's clumsy and adventurous father.
- Stella, Arnold's intelligent and daring mother.
- Grandpa's father
- Grandpa's grandfather
The Sunset Arms boarders and their families
- Oskar Kokoshka, a whiny, lazy and self-centered immigrant from Czechoslovakia.
- Suzie Kokoshka, Oskar's diligent, loving and patient wife.
- Nancy and Baby Oskar, Suzie's cousin and her child (Nancy did not reside in the Sunset Arms and points out bitterly that Baby Oskar is NOT named after Kokoshka).
- Ernie Potts, a short-statured (and tempered) demolition worker.
- Mr. Hyunh, a quirky, sometimes odd immigrant from Vietnam, working in a Mexican restaurant.
- Mai Hyunh, Mr. Hyunh's long lost daughter (never resided in Sunset Arms).
- Mr. Smith, a mysterious person who never speaks to the rest of the boarders (in the first season only).
- Lana Vail (in the first season only).
- Mr. Purdy (in the first season only; voiced by Joseph Purdy himself).
For short time, the following people lived at the Sunset Arms: Gerald Johanssen ("Gerald Comes Over"), Rhonda Wellington Lloyd, Brooke Lloyd, Buckley Lloyd ("Rhonda Goes Broke"), Dino Spumoni and Don Reynolds ("Partners").
Arnold's classmates and their families
- Helga G. Pataki, a domineering, intimidating and tomboyish bully who hides an intelligent and poetic side as well as an obsession with Arnold.
- Robert "Big Bob" Pataki, Olga and Helga's rough, intimidating and sometimes neglectful father.
- Miriam Pataki, Olga and Helga's drowsy and inattentive mother; she is an alcoholic who constantly passes out and sleeps around the house all day.
- Olga Pataki, Helga's ditzy, yet sweet and academically-gifted "perfect" big sister.
- Gerald Johanssen. Arnold's cool, street-smart, and loyal best friend.
- Martin Johanssen, Jamie O, Gerald and Timberly's father. While a cheap businessman, he is a caring and sensible father.
- Mrs. Johanssen, Jamie O, Gerald, and Timberly's mother, works as a cashier at the Corner Store. She is a kind, devoted mother.
- Jamie O Johanssen, Gerald and Timberly's older brother; he loves to pick on Gerald.
- Timberly Johanssen, Jamie O and Gerald's little sister; a brat who often uses deception and manipulation to get her way then breaks down into sob stories to get out of all blame (Arnold and Gerald end up being able to use the latter to their advantage in 'Chocolate Turtles').
- Harold Berman, a dim-witted fat kid/occasional bully.
- Jerry Berman, his father, sometimes hard on him but is trying to be a supportive father.
- Marilyn Berman, his mother who often spoils Harold.
- Harold's aunt (name unknown)
- Phoebe Heyerdahl, Helga's shy, intelligent and docile best friend; she is the smartest kid in Mr. Simmons class.
- Kyo Heyerdahl, her father.
- Reba Heyerdahl, her mother.
- Rhonda Wellington Lloyd, a pompous rich girl and self-proclaimed fashion queen.
- Brooke Lloyd, her mother.
- Buckley Lloyd, her father.
- Lila Sawyer, a demure and compassionate young country girl Arnold held a crush on.
- Stinky Peterson, a lanky, somewhat simple boy whose family originally hails from Arkansas.
- Stinky's father
- Stinky's grandfather
- Stinky's grandmother
- Uncle Stinky
- Squeaky Peterson, most likely Stinky's cousin ("False Alarm").
- Eugene Horowitz, the optimistic and friendly class jinx.
- Nate Horowitz, his father.
- Mrs. Horowitz, his mother.
- Sid, a boy with paranoid tendencies.
- Ray, his father
- Sid's mother (Only appears in "Parents' Day" and "On the Lam" )
- Thaddeus "Curly" Gammelthorpe, the class psychopath.
- Torvald, the biggest boy in class who was tutored by Arnold.
- Sheena, a girl who doesn't like violence.
- Sheena's uncle Earl, a boatman
- Sheena's aunt Shelley, the school nurse
- Lorenzo, a rich Hispanic-American boy who doesn't have enough time to be a kid.
- Brainy, a geek, Helga's stalker.
- Nadine, young entomologist, Rhonda's best friend.
- Park, a Korean American boy in Arnold's class.
- Robert, a Kid who is seen in the background of Arnold’s class he has blonde hair and Glasses
- Peapod Kid (He got his name from the episode "Downtown as Fruits", because he was a peapod in the play.)
- Joey Stevenson
Grandpa Phil was also was Arnold's classmate in the episode "Back to School".
Other students of PS 118 and their families
- Billy
- Burt, Connie's boyfriend
- Big Gino, the boss of the school mafia
- Campfire Lass, a girl with a Scottish accent who sells chocolate turtles
- Cookie, a sixth grade girl
- Chocolate Boy, a boy obsessed with chocolate
- Connie, a sixth grade girl who "dated" Gerald; Maria's best friend
- Edmund, Wolfgang's dim-witted sidekick
- Gloria, a girl who looks like Helga, but has a nicer personality
- Katrinka
- Ludwig, a bully, Wolfgang's rival
- Marcy
- Maria, a sixth grade Hispanic American girl who "dated" Arnold; Connie's best friend
- Mary, one of Big Patty's victims
- Mary Margaret, computer club president ("Arnold's E-Files")
- Mickey the Weasel, a duplicitous fifth grader
- Mike Smedvick, a former P.S. 118 student who was sent to juvenile hall
- Patricia "Big Patty" Smith, a sixth grade bully, Harold's possible love interest
- Ruth P. McDougal, a sixth grade girl, Arnold's crush
- Seymour Stump, cheating contestant
- Siobhan, an incredibly smart third grade girl
- Simone, a sixth grade girl
- Tommy, Maria's boyfriend
- Wolfgang, the main bully of the block
Workers of PS 118 and their families
- Dr. Bliss, a child psychologist who worked in the school for a day and later became Helga's psychoanalyst
- Miss Felter, a substitute teacher, Arnold's crush
- Mr. Frank, a burnt-out sixth grade teacher
- Lieutenant Major Goose, Arnold's teacher for short time (after Miss Slovak and after Arnold's class scared Mr. Simmons off), a martinet. He also was Martin Johanssen's drill sergeant during Vietnam War in "Veterans Day".
- Lunch lady
- Mr. Packenham, a fifth grade teacher and opera enthusiast
- Sheena's aunt Shelley, the school nurse (already mentioned above)
- Mr. Robert Simmons, Arnold's new teacher (since second season)
- Miss Slovak, Arnold's former teacher (in the first season)
- Principal Wartz
- Coach Jack Wittenberg, a highly competitive sports coach (voiced by James Belushi)
- Tish Wittenberg, his equally competitive wife, the coach of girls teams
- Tucker Wittenberg, his son, a student of PS 118
- Farcus, the school janitor ("Hookey")
Olga Pataki was Mr. Simmons's assistant in the episode "Student Teacher".
Other characters and their families
- Agatha Caulfield, Arnold's favorite author, later became disillusioned and stopped writing
- Alphonse Perrier du von Scheck, a developer who tried to tear down Arnold's neighborhood (Vine Street) so he could build a mall (Hey Arnold!: The Movie)
- Arnold Skelter, Miss Felter's boyfriend
- Mr. Bailey, a cranky office clerk working at the Hall of Records
- Betty, Mr. Leichliter's old love
- Blind Man, A blind man who is riding the subway in the episode "Das Subway"
- Bridget, head of a secret organization (Hey Arnold!: The Movie)
- Caesar, a man who seems to have many jobs
- Mr. Camacho, the new owner of El Patio, the restaurant where Mr. Hyunh works ("Family Man")
- Camille, Johnny Stitches's assistant
- Candy Maldonado, Dino's ex-wife
- Jimmy Maldonado, Candy's son
- Carlos, Dance instructor ("Summer Love")
- Cecile, Arnold's pen pal from France
- Cherice, a manipulative girl Jamie O falls in love with in the episode "Jamie O in Love"
- Chloe, the girl Gerald had a crush on, while she felt those feelings for his brother, Jamie O
- Councilman Gladhand, a former member of the city council, defeated by Marty Green
- Councilman Marty Green, a butcher in Arnold's neighborhood and later city council member
- Dan and Sherm, Mr. Green's friends and fellow butchers ("Freeze Frame")
- Daniel Newton, documentary film director
- Davy Jones, a singer (voiced by Davy Jones himself)
- Dino Spumoni, a formerly famous singer
- Don Reynolds Dino Spumoni's former partner who wrote the lyrics for all of his songs
- Dr. Iron Claw, a character invented by Grandpa Phil for one of his stories about Arnold's Parents.
- Dr. Murray Steiglitz, a physician at Drymon Clinical Hospital
- Eduardo, best friend of Arnold's father, Miles, who asked him and Stella to come to San Lorenzo to help the Green Eyed People
- Floyd, an owner of a bar in countryside
- Frankie G. and his gang: Tony B., Philly D., Joey C., and Chas, are a group of burglars
- Giuseppe, a go-kart driver
- Grubby, a homeless man who lives on a subway train in the episode "Das Subway
- Harvey, the neighborhood mailman
- Hilda, Fifi, Rhoda, Harry, Stumpy, Gerard, Kid, Lulu, kids (likely imaginary) who live in Arnold's cousin Arnie's hometown, and Helga, Phoebe, Rhonda, Harold, Stinky, Gerald, Sid, and Lila's respective doppelgangers and characters of Arnold's dream ("Arnold Visits Arnie")
- Hillary, a woman who appears on the subway train in the episode "Das Subway"
- Inge, a German nanny hired by the Patakis in the episode "Helga and the Nanny"
- Jacques, waiter at the Chez Paris restaurant
- Jimmy Kafka, Grandpa Phil's childhood best friend and rival
- Johnny Stitches, an arrogant fashion designer
- Lasombra, a river pirate who steals and sells ancient treasures throughout Central America
- Leichliter, a dramatic critic and director
- Lola, Ernie Potts's girlfriend
- Lorraine, Councilman Gladhand's secretary
- Lucky, an unlucky fireman
- Ronnie Matthews, Phoebe's favorite pop star who turns out to be a fraud
- Maurice, actor who plays Eugene's hero "The Abdicator"; the star of a titular TV show
- Madame Blanche, a gypsy scam-artist who sells magic potions
- Madame Parvenu, owner of a finishing school
- Mayor Dixie, the mayor of Hillwood
- Mickey Kaline, a retired baseball player
- Mona, Murray's girlfriend (Hey Arnold!: The Movie)
- Monkeyman, a lanky young man who fights crime on the streets
- Mr. Wacko, a businessman who sells wrist watches
- Mrs. Ryle, the elderly owner of Cupcake or "Thurston" (Harold's Kitty)
- Mrs. Vitello, a flowershop owner in Arnold's neighborhood
- Murray, a cynical bus driver (Hey Arnold!: The Movie)
- Ned, a Radio DJ
- Nick Vermicelli, Big Bob's business advisor, a shady individual with implied criminal ties
- Officer Pudney, a police officer in Hillwood
- Patrice, Charice's friend
- Pet Shop Cashier, sarcastic teenaged girl that works at Pet Love
- Pigeon Man, a gentle outcast who prefers to associate with pigeons rather than people.
- Pinkerton, a former employee at Big Bob's Beepers
- Pop Daddy, the character titular main character of Gerald and Arnold's favorite television show.
- Rabbi Goldberg, the Bermans' rabbi
- Ray Doppel, an impersonator of Dino Spumoni, and the star of Dino Mania
- Red, Mr. Scheck's assistant (Hey Arnold!: The Movie)
- Revis and Jackie Lee, talk-show hosts
- Rex Smythe-Higgins, old rival of Grandpa Phil
- Rex Smythe-Higgins III, his grandson, Arnold's occasional rival
- Robbie Fisher, an arrogant Chinese Checkers champion who caused Grandpa "Steely" Phil to retire from the game some 40 years before their rematch in the episode "Steely Phil"
- Sammy Redman, a rich man who befriends Arnold
- Alan Redman, his son who is an amateur photographer
- Sandy, Summer's boyfriend in the episode "Summer Love"
- Sewer king, an insane hermit who lives in the city's sewer
- Seymour, AKA "The Disposal", a contestant in the "Eating Contest"
- Slim, a skilled mechanical bull rider
- Stoop Kid, a teenager who lives on a stoop and was afraid to leave it until Arnold helps him conquer his fears
- Summer, a girl Arnold who manipulated Arnold and tricked him into thinking that she had a crush on him ("Summer Love")
- Superintendant Chaplin, an official who supervises schools in Hillwood
- Vic and Morrie, Oskar's friends with shady reputations
- Willie, the Jolly Olly Man, an irate, mentally unbalanced ice cream truck driver.
- Wyatt, an ice cream server at Slausen's Ice Cream Parlor
- Zamboni Jones, a psychic
Animals
- Abner, Arnold's pet pig. Was a gift from the Green-eyed People at Miles and Stella's wedding from a water basket. Phil first thought to cook Abner but Stella decided to keep the pig
- Angel, Angel fish #2108 adopted by Eugene.
- Big Caesar, the legendary giant fish that lives in City Pond
- Chester, Arnold's carrier pigeon
- Cupcake a.k.a. Thurston, stray kitten Harold was refusing to give back to its owner
- Elbert, Eugene's imaginary pet hippo.
- Glue Boy, racing mule bought by Ernie, Oskar, and Mr. Hyunh who thought that it was a horse; Glue Boy's favorite song is "You Are My Sunshine".
- Helga's parrot, who was bought by Big Bob, lived at Arnold's, and was eaten up by Phoebe's monitor lizard
- Henry, Eugene's pet fish killed by Arnold with a yo-yo
- Lock Jaw, an old, tired, graffiti-covered turtle that lived in the city Aquarium
- Lurkey, Stinky's father's pet turkey, referred to as "Lurkey the Turkey", who came to an untimely fate when Stinky's family killed and ate him.
- Monitor Lizard, Helga's unwelcome gift to Phoebe
- Pinky, Billy's pet worm, inadvertently stepped on by Helga
- Pooter (1924-1927), a dog that Grandpa Phil and Aunt Mitzi had when they were young, and whose death caused them to refuse to speak to each other for 71 years.
- Snappy, Stinky's turtle who was used for an April Fool's Day prank
- Sidney, Sid's pet frog
Phantoms
- Four-Eyed Jack, a former Sunset Arms resident who haunts the Boarding House
- Ghost Bride (Cynthia Snell), who killed her bridegroom and sister and now haunts the city cemetery
- Haunted Train and its mad conductor, that once a year appears on an abandoned train station
- Headless Cabbie, who gives rides in the city park
- Laughing Lady, a mysterious phantom who was responsible for the death of the Headless Cabbie ("Headless Cabbie (episode)")
- Wheezin' Ed, a counterfeiter from the Prohibition era who now haunts Elk Island
Trivia and facts
- Oskar Kokoschka is also the name of an Austrian painter.
- Ludwig and Wolfgang are the names of the two "bullies" in the show. The names come directly from arguably the two most famous composers of all time — Ludwig van Beethoven and Wolfgang Amadeus Mozart.
- Gerald's last name is spelled "Johanssen", "Johanson", and "Johansson" in the show. Phoebe's and Stinky's last names have two variants: "Heyerdahl" / "Hyerdahl" and "Peterson" / "Petersen" respectively. Suzie Kokoshka's first name was also spelled "Susie", and Timberly's as "Timberley".
- Fuzzy Slippers, Gerald's mysterious informant, never was seen in the show.