Welcome Back, Kotter

• Played by Gabe Kaplan

Gabe Kotter is a flippant but well-meaning teacher who returns to his alma mater, James Buchanan High, to teach a group of remedial students known as the Sweathogs. Kotter has a unique insight of the potential of these purportedly "unteachable" pupils, as well as the difficulties and scrutiny they encounter on a daily basis, as he was a "founder member" of the original Sweathogs (educational, social, and emotional). On his first day on the job, he launches into a Groucho Marx impersonation. Kotter is married to Julie throughout the series; they eventually have twin girls, Robin and Rachel. It is confirmed by Julie in the episode "Follow the Leader (part 1)" that Gabe is Jewish. During season four, Kaplan had contract issues with the executive producer, which resulted in Kotter's character appearing in only a handful of episodes. In season four, the invisible principal John Lazarus retires, and Kotter becomes the vice-principal. Though he is said to maintain some social studies training duties, most of that season's shows are filmed outside his classroom (#11) or, if in Room 11, Mr. Woodman is teaching. To minimize Kotter's absence, scenes were shot in either the school's hallway, the schoolyard, or the principal's waiting area. Season four ended the series.

• Played by Marcia Strassman

Julie Kotter is Gabe's wife and closest friend. Though she has a sense of humor, she often wishes Gabe would take matters more seriously. She is occasionally upset with the amount of time her husband spends with his students, and she is troubled that he allows them to visit their apartment regularly; in the two-part story arc "Follow the Leader", the Sweathogs' constant intrusions lead Julie to separate briefly from Gabe and even seriously consider divorce. Originally from Nebraska, with a college degree in anthropology, Julie eventually becomes a secretary at Buchanan, and later a substitute teacher after Gabe's promotion to vice-principal. She makes several references to her "world famous tuna casserole", a common meal at the Kotter dinner table, which Gabe and the Sweathogs deem inedible.

• Played by John Sylvester White

Michael Woodman is the curmudgeonly vice-principal (and later principal) of Buchanan High. He makes no secret of his dislike for the Sweathogs, whom he considers the bottom of the social stratum at his school. He refers to non-Sweathogs as "real" students. When Kotter was a student at Buchanan, Woodman taught social studies, the same class Kotter returns to teach at Buchanan. His old age, and sometimes his diminutive height, are common jokes with the Sweathogs. Woodman opposes Kotter's unorthodox teaching methods, and at one point even puts Kotter in front of the school's review board in an unsuccessful attempt to have him fired. As the series progresses, Woodman begins to tolerate them marginally. In the season one episode "No More Mr. Nice Guy", Woodman is shown to be a gifted teacher, willing to wear historic costumes and role-play in front of the class during his lessons.

• Played by John Travolta

Vinnie Barbarino is a cocky Italian-American, the "unofficial official" leader and resident heartthrob of the Sweathogs. He has a need to be the center of attention, as seen when he admits to making it rain in the school gymnasium. In the two-episode "Follow the Leader", Barbarino quits the Sweathogs and drops out of school in anger when Freddy Washington is chosen as the "leader" of the group, though he returns as leader at the finish of the story. Barbarino's prowess with women is sometimes a source of envy and more often, amusement among his classmates. On occasion, he breaks out in a song about his last name sung to the tune of "Barbara Ann". He was the first of the Sweathogs to move out on his own, when he got a job as a hospital orderly. In the first episode of the series' fourth season, he has a girlfriend, Sally. Vinnie is Catholic (often describing his mother as a saint), and, as shown in "I'm Having Their Baby", is a Star Trek fan. Little is known about Vinnie's home life, other than that his parents argue a lot ("Follow the Leader (Part 2)") and take turns beating him when in a mutual rage. His mother's name is Margie ("The Great Debate"), whom he describes in the pilot episode as “holy” and he shares a bed with his brother. The episode "Don't Come Up And See Me Sometime" implies that Vinnie is the older of the two. The character is seen less frequently in season 4, appearing in only 10 of the first 15 episodes of the season, before leaving the series entirely.

• Played by Ron Palillo

The class clown of the Sweathogs, Arnold Horshack, is completely comfortable with his oddball, if naïve, personality. Horshack was known for his unique observations and his wheezing laugh, similar to that of a hyena. (Palillo revealed on a 1995 episode of The Jenny Jones Show that it originated from the way his father breathed during the last two weeks of his life as he lay dying from lung cancer.) It is possible that academically he is the smartest Sweathog. He is the only central Sweathog character to be promoted out of a remedial academics class, but he soon returns after feeling out of place. He has an affection for acting and enjoys old movies, particularly 1930s musicals. He eventually marries Mary Johnson, a co-worker and fellow Sweathog. Although his surname sounds like a term for a brothel, he claims it is a "very old and respected name" meaning "the cattle are dying." His middle name (and his mother's maiden name) is "Dingfelder".

• Played by Lawrence Hilton-Jacobs

Freddie Washington is the hip black student known as the athletic Sweathog for his skills on the basketball court (although in the episode "Basket Case", Mr. Kotter almost beats Freddie in a one-on-one game). Washington claimed his nickname came from his habit of "pretending to play the bass" and singing "Boom-boom-boom-boom!". His trademark phrase is, "Hi, there" (spoken with a deep voice and a broad smile) and he calls Mr. Kotter "Mister Kot-TAIR". Though often the voice of reason among his classmates, Washington nonetheless is a willing participant in the Sweathogs' various antics and pranks. Freddie also finds success as a radio disc jockey along with another former Sweathog, Wally "The Wow" Wexler (played by George Carlin). At one point, Washington challenges Barbarino for leadership of the Sweathogs and even replaces him for a time until the group grows tired of his dictatorial style.

Washington has an older sister, who got divorced twice while living in Vermont ("The Longest Weekend"), and a brother, Leroy. In "The Great Debate", it is revealed that he has another brother, Douglas, and that his father's name is Lincoln, portrayed by R&B singer Carlton "King" Coleman. Kotter uses his own past to bond with Freddie, because, in addition to being a former Sweathog he was also a former star of Buchanan's basketball team.

• Played by Robert Hegyes

Epstein is a fiercely proud Puerto Rican Jew. When asked if his mother was Puerto Rican, Juan replies that his mother's maiden name was Bibbermann and that his grandfather "saw Puerto Rico from the ship as he was making his way to America and decided to settle there instead of Miami", making him one of the earliest Puerto Rican Jews. Juan is thus Puerto Rican on his father's side and Jewish on both parents' sides.

He is one of the toughest students at Buchanan High, despite his short stature. He normally walks with a tough-man strut and was voted "Most Likely to Take a Life" by his peers. In the season one episode "One of Our Sweathogs Is Missing", Epstein was said to be the sixth of ten children (when speaking on the phone to his mother, who had failed to notice that he had been missing for three days, she apparently failed to recognize his name and he had to further identify himself as "Number Six"), although he later mentions, in "I'm Having Their Baby", that his mother only gave birth eight times, implying two of them were twin births. Only four of his siblings are mentioned by name: his brothers Pedro, Irving, and Sanchez ("One of Our Sweathogs Is Missing"), establishing that some of his siblings had Jewish names and others Puerto Rican names, and a younger sister, Carmen ("A Love Story"). Epstein's toughness was downplayed later on, and he became more of a wiseguy. He was also known to have a "buddy" relationship with Principal Lazarus, as he often referred to him by his first name, Jack. On a few occasions, when Kotter did his Groucho Marx impersonation, Epstein would jump in and impersonate Chico Marx or Harpo Marx. Epstein's diminutive height, large hair, and fake excuse notes (always signed "Epstein's Mother") were running gags.

• Played by Debralee Scott

Rosie Totsie is the femme fatale purported to have put the "sweat" in Sweathog, though her reputation is largely exaggerated by the Sweathogs' word of mouth. Her promiscuity is at least in part a reaction to the strict discipline enforced by her father, the Reverend Totsie. To restore her good name, and to prove a point, she fabricates a story about one of the Sweathogs getting her pregnant. The character was a favorite among male viewers but was phased out of the series at the end of the first season when Scott was picked to co-star in the syndicated Norman Lear comedy Mary Hartman, Mary Hartman. She reprised the role in a 1978 episode, "The Return of Hotsie Totsie", in which it was revealed that she dropped out of school because she became pregnant and had to become a stripper to support her infant child.

• Played by Helaine Lembeck

A recurring non-Sweathog character in the earlier seasons, Borden is a straight-A student and editor of the Buchanan Bugle, the school newspaper. She was Barbarino's tutor, and even dated him at one time. Despite her academic superiority, she can easily hold her own in a Dozens contest against any Sweathog.

• Played by Stephen Shortridge

Introduced as a regular character in the fourth and final season, Beau is a handsome, friendly, blond, silver-tongued southerner who transfers from New Orleans after being kicked out of several other schools. He ends up in Kotter's class. The producers sought a heartthrob who was not a direct knock-off of the "Italian Stallion" trend that was permeating Hollywood in the mid-1970s, and who would improve ratings in the South, where the show's New York setting was seen as unrelatable. They wanted to retain female viewers but avoid a Travolta clone. Beau's first reaction to the term "Sweathog" was "That sounds gross." He seems to have a way with women, as shown in later episodes. One of his running jokes involves imparting esoteric and nonsensical sayings, such as "a real man never steps on a pregnant alligator".

DC Comics published ten issues of a Welcome Back, Kotter comic book starting in 1976.[6] Following its cancellation in 1979, a Limited Collectors' Edition was issued, incorporating a four-page "On the Set" section and photographs from the show.[7]

A series of novels based on characters and dialog of the series was written by William Johnston and published by Tempo Books in the 1970s.

Mattel produced a series of 9-inch Welcome Back, Kotter action figures in 1977. Figures produced included Barbarino, Horshack, Epstein, Washington, and Mr. Kotter.[8]