Gabriel "Gabe" Kotter
The main protagonist and school teacher. A former Sweathog who made good and is coming back to his old Alma Mater to teach.
- Dean Bitterman: After he becomes vice-principal in Season 4, the jokes are less frequent as he takes his new position more seriously.
- Demoted to Extra: After Season 3 ended, the writing staff was fired, and writers from older sitcoms were brought in to try and boost the low Nielsen ratings. Off-screen disputes resulted in Gabe breaking his contract and reducing his appearances in Season 4.
- The Mentor: To the Sweathogs. This even causes tension between him and Julie in a two-parter episode of the first season.
- Parental Substitute: Often played this role with the Sweathogs, especially Horshack.
- The One Who Made It Out: But he came back to teach a new generation of Sweathogs.
Gabe Kotter's long-suffering young Mid-Western wife from Nebraska.
- A Degree in Useless: Has an Anthropology degree, which isn't much when you're a homemaker who lives in inner city of Big Applesauce. She even hangs a lampshade on this in a first-season episode.
- Cordon Bleugh Chef: She puts stuff like prunes and coffee beans in the tuna casserole. It doesn't go over well.
- Deadpan Snarker: To balance out her husband's terrible jokes.
- House Wife: Starts out as this. Eventually becomes a secretary and teacher.
- Straight Man and Wise Guy: A rare romantic example, she's often the straight man to Gabe's wise guy.
- Sugar-and-Ice Personality: Has this towards the Sweathogs initially, but eventually becomes a motherly figure to them the way Gabe's like their father.
Vincent "Vinnie" Barbarino
Sicilian-American self-proclaimed Leader of the Sweathogs. A poor student but a big-time hearthrob who's also a bit of a big-time doofus.
- Berserk Button: Whenever other students insult him, resorting to Your Mom jokes is going too far; Vinnie will stand up and say that she's a saint.
- Brainless Beauty: Even by Sweathog standards, Vinnie isn't too bright. The girls don't seem to mind.
- It's All My Fault: Feels this way when the shop teacher dies of a heart attack while yelling at him. He even turns himself in to the police as a murderer!
- The Leader: Of the Sweathogs.
- Mama's Boy: He loves his mama, insulting her is a huge Berserk Button for him.
Arnold Dingfelder Horshack
A strange, diminutive, Polish-American oddball and a bit of a sidekick to Barbarino.
- Cloud Cuckoo Lander: In spades!
- Determinator: In "Horshack vs. Carvelli", he volunteers to take on Carvelli in a boxing match, with the other Sweathogs and Mr. Kotter trying to talk him out of it to no avail. When Wooodman gets word that Horshack is the one that volunteers for the fight, he roots for Horshack in the hopes that Buchanan will get bragging rights. Even though he gets knocked out by Carvelli, he gains the Sweathogs' respect for his valiant efforts.
- Hidden Depths: It turns out he's actually a fairly good student when he applies himself; he is even placed with regular mainstream classes in one episode. He struggles with being separated from the other Sweathogs and is brought back to Mr. Kotter's class.
- In the final season, he successfully manages to talk down the suicidal Mary Johnson when even Julie and Mr. Woodman fail.
- Honorable Marriage Proposal: Proposes to Rosalie when she's trying to guilt-trip the other boys with a pregnancy scare (she made it up). It earns him a second date once she drops the charade.
- Kavorka Man: Downplayed; even though he's though he's geeky and somewhat unattractive, and doesn't have a ton of success with the ladies, he still manages to get lucky once in a while at first, offering to be a fatherly figure to Rosalie's baby in Season 1 (which was part of Rosalie's scheme to teach the other Sweathogs a lesson) and later in Season 3 when Rosalie had a baby for real. He manages to get a good-looking date in Season 2's "Sadie Hawkins Day", goes on a date with Juan's sister Carmen, falls for Julie after several rejections, and eventually helps save the life of Mary Johnson, whom he eventually marries.
- Lovable Nerd: The geekiest of the Sweathogs who feels uncomfortable in any other classroom, preferring to hang out with the Sweathogs.
- Promotion to Parent: Tries to do this in "Whatever Happened To Arnold" part 2 after his fifth stepfather dies. He seems successful at the end, but little more is said about this.
- Tagalong Kid: In the beginning. He comes into his own in later seasons and is the first of the Sweathogs to marry.
- Teacher/Student Romance: A teacher's wife variant; Arnold has a brief crush on Mrs. Kotter in "Horshack and Madame X", when Julie offers him words of comfort after failing to be noticed by the other girls at Buchanan High. By the end of the episode, Julie and Arnold decide to discontinue while remaining friends, out of respect for Mr. Kotter.
- Teens Are Short: Amongst the Sweathogs, Arnold and Juan Epstein are relatively shorter than Vinnie and Freddie, who are definitely taller than them.
Freddie "Boom Boom" Washington
A tall, wise-cracking African-American "cool dude" who's also a school basketball player.
- Dumb Jock: Is a star basketball player but gets terrible grades.
- Embarrassing Middle Name: When filling out an application in Part 2 of "The Drop-Ins", his middle name is revealed as Percy.
- Starcrossed Lovers: How he and Verna Jean saw themselves.
- The Lancer: Especially to Vinnie. Becomes The Leader in the 4th season, after Vinnie's appearances become more sporadic.
Juan Luis Pedro Phillipo de Huevos Epstein
Short, pugnacious Puerto-Rican Jewish guy who is famous for writing his own hilarious B.S. excuse notes.
- Bruiser with a Soft Center: He's a good guy deep down, and he's very good with animals.
- Face of a Thug: When he goes missing, one episode, the Sweathogs create a poster complete with "Have you seen this?"
- Informed Judaism: In one episode, when asked if his mother was Puerto Rican, he replies that his mother's maiden name was Bibberman.
- My Sister Is Off-Limits: Gets mad when Freddie and Vinnie attempt to date Carmen, the only reason he gives Arnold his blessing is because he thinks Arnold is "safe". Luckily, Carmen calls him out for it and he admits it's his only way of showing he loves her.
- The Napoleon: It is not a good idea to get Juan mad, especially since he was voted "most likely to take a life" by his peers.
- The Pig-Pen: His gym locker is considered a major hazard and often used for a threat against people the Sweathogs dislike.
- Twofer Token Minority: A Puerto Rican Jew.
Female Sweathog and Proud of it.
- Chuck Cunningham Syndrome: Gone by the end of Season 3.
- Cousin Oliver: She appears in the second half of the third season for seven episodes and then is never heard from again, presumably as a result of the writing staff being fired after Season 3 and replaced with veteran scriptwriters who had worked with older sitcoms.
- You Go, Girl!: Only with being a Sweathog rather than sports.
A pretty blonde student and one-episode Sweathog who's not what she seems.
- Chuck Cunningham Syndrome: In only one episode as an actual character. In others, she's just some random student.
- Distracted by the Sexy: The other Sweathogs go crazy over her when she shows up in class on her first day.
Beauregarde "Beau" DeLabarre
Southern Cajun boy, replacement-Sweathog for Barbarino.
- Cousin Oliver: After Gabe reduced his appearances in the fourth season and John Travolta focused more on his movie career, Beau helped to fill some of the void, along with Carvelli and Murray transferring to Buchanan.
- I Just Want to Have Friends: It's revealed he's been shuffled from school to school and relative to relative, and that his slick Southern Gentleman personality is mostly an act he puts on to charm people into liking him. Later, he's so desperate to keep the Sweathogs as his friends that he plays a prank on Mr. Woodman just to impress them.
- New Transfer Student
- Rule of Three: His main gag is to make "there are three things" lists pertaining to a situation. Only one of those three is relevant. Julie even calls him on this in his debut episode.
Rosalie "Hotsie" Totsie
The main female Sweathog of the first season (along with Verna Jean). A redhead with a rumored "bad reputation".
- The Bus Came Back: And now she's a stripper who dropped out of school due to a real Teen Pregnancy.
- Chuck Cunningham Syndrome: She drops out of school after Season 1; in a Season 3 followup, Gabe and the Sweathogs help her get back into school to finish her education.
- Foreshadowing: In "Whodunit?", Rosalie fakes a Teen Pregnancy to show the other students that she's not a loose and easy girl. Later, in "The Return of Hotsy Totsy", after she's dropped out of school and has mothered a child for real, Arnold and the other Sweathogs help her return to Buchanan so she can resume her education and improve her career prospects.
- Really Gets Around: Is wrongly thought to be this & subject to rumors in Season 1; she fakes a Teen Pregnancy to make the guys admit otherwise.
Female Sweathog and Freddie Washington's girlfriend. She eventually moves away to Staten Island.
- Only Sane Man: She was probably the closest thing to the reasonable straight man among the Sweathogs.
- Star-Crossed Lovers: Freddie treats the idea of her moving away like they're about to become this, even trying to marry her to prevent it.
A dopey neighborhood bully.
- Ascended Extra: He transfers to Buchanan High as a regular student in the fourth season.
- Hidden Depths: Proved to be a gifted guitar player and singer during a school talent show in the last season.
- Sitcom Arch-Nemesis: Frequently antagonises the Sweathogs.
- Ascended Extra: Murray transfers to Buchanan High alongside his friend Carvelli.
- Mad Libs Catchphrase: "Did you hear dat? My man Carvelli said [crack Carvelli just made]."
The unseen Principal. Is on very chummy terms with Juan Epstein, much to the chagrin of Mr. Woodman.
Vice Principal Michael Woodman
The main antagonist. An old grump with a very dim view of all things "Sweathog".
- Genius Ditz: In "No More Mr. Nice Guy", he takes over Kotter's class and does an exceptional job teaching the students with dramatized versions of the American Revolution.
- Jerk with a Heart of Gold: He's actually helped the Sweathogs a few times, though it's extremely rare; he helps the Kotters move into their new apartment after the twins are born, and when Barbarino moves into his own apartment, he takes part in the housewarming party. He and Kotter also seem to have a Vitriolic Best Buds thing going on.
An incredibly shrill, snooty, snotty, loud but very studious young student. Often an antagonist towards the Sweathogs.
- Abhorrent Admirer: To Vinnie, though this is downplayed in that he seems to like her back at times.
- Cute, but Cacophonic: Emphasis on the "Cacophonic" part. Like scratching a chalkboard.
Another super-studious but insufferable fellow student & antagonist. Is also a very hot-tempered, physically and emotionally aggressive jerk who even defeats Juan Epstein in a fist-fight.
An unseen female teacher who's mentioned often in passing as a gag. Presumably an elderly woman.
- New Job as the Plot Demands: The teacher version. Depending on the episode, she's either described as the Social Studies teacher, Art teacher or something else.