T.J. Henderson

The lead character (played by Tahj Mowry), T.J. Henderson (whose full first and middle name were not revealed) is the youngest child in his family, portrayed as 10 to 11 years old in seasons 1 and 2, and 12 in season 3. As the show's title suggests, T.J. exhibits exceptional intelligence with an IQ of 180, knows several different languages and has a photographic memory. He skips six grades from 4th to 10th grade due to his intelligence, after his teachers at his old school discovered that T.J. was not feeling challenged enough by a 4th grade curriculum. Outside of the occasional moments of awkwardness that are bound to occur when a child interacts with high schoolers, T.J. has a peaceable existence in the school, and is eager to be involved in school functions: so much so, that he becomes the mascot for his school basketball team ("Pugnacious Pete, the Powerful Piedmont Penguin"), with mixed results.

T.J. is later promoted to being the equipment manager for the Piedmont High Penguins basketball team. He spends a fair amount of time at school with his older brother, Marcus, and his brother's best friend, Mo Tibbs. Although they have some tense moments, the two brothers, who share a bedroom throughout the series, care for each other deeply. Though T.J. spends much of his time around teenagers, he is occasionally seen spending time with children his own age, which occasionally results in his attempts to try to fit in with them in situations where he feels out of place. T.J. plays keyboard for Marcus' band Mackadocious in several episodes, after one of the group's other members breaks his finger in an accident (he also briefly served as the band's manager in the season three episode "Achy Breaky Heart").

Marcus Henderson

Played by Jason Weaver, the character of Marcus Henderson is T.J.'s older brother and the second-oldest child in the Henderson family (behind sister Yvette). He is a fairly popular student at Piedmont High and reacts remarkably well to his younger brother sharing nearly every one of his classes, although tensions do arise from time to time (this is especially true in the pilot episode, to the point where he tells T.J. that he wishes that his younger brother would disappear from his life). He gets average or below average grades, simply because he does not apply himself. He is a forward on his school's basketball team and has several love interests throughout the run of the show, none of which are seen for more than one episode. He is also the lead vocalist for the band which he created, Mackadocious, in which his best friend Mo is the bassist and T.J. is the keyboardist.

Tasha Yvette Henderson

Played by Essence Atkins, the character of Tasha Yvette Henderson – who is generally referred to by her middle name – is the older sister of Marcus and T.J. and the only daughter of Floyd Henderson. She is in the eleventh grade in season one and graduates high school at the end of season three. Yvette is intelligent, is usually level-headed, is a women's rights activist and has a passion for art, including drama, photography and dancing. She is also the editor of the school newspaper, The Penguin (a position that is the partial basis of "Stop the Presses", in which T.J. decides to start his own newspaper – The Weekly Veritas, which he abruptly switches from a hard news to a tabloid publication upon the advice of Marcus and Mo due to flagging sales – after he becomes dissatisfied with repeatedly assigned by Yvette to write puff pieces). Yvette sometimes finds it hard being the only girl in a family full of mostly men, as seen in the season two episode "Men Working Badly".

Episodes in which her character dates a fellow Piedmont High student feature Yvette sometimes overthinking the relationship, such as when she dated a dimwitted swimmer whom she had mainly a physical attraction to in the episode "Big Picture" or when high school senior Yvette dated a sophomore in the episode "T.A. or Not T.A." In the season one episode "The Code", it is revealed that she has a fake I.D. that says she is 28 years old. After being rejected from Princeton University, Yvette planned to stay in Washington, D.C. to attend Georgetown University as the series ends. Although the character of Yvette is in her late teens, Essence Atkins (born on February 7, 1972, and therefore being in her mid-20s during the run of the series) is actually only ten years younger than John Marshall Jones, who plays her character's father Floyd.

Floyd Henderson

Played by John Marshall Jones, the character of Floyd Henderson is the strict widowed father of Yvette, Marcus, and T.J. He is a self-made businessman and owns a roofing company called Floyd Henderson Contracting. After the loss of his wife, he would eventually start dating once again: such as in season two's "Dateline", when T.J. sets Floyd up with a woman named Jamie (Jackie Mari Roberts) through an online dating service, whom T.J. eventually gets jealous of after Floyd spends less time with him; and in the season three episode "Beating is Fundamental", when he dates a woman whose son T.J. dislikes (and later punches), though many of the women that he is seen dating in the series are usually not seen for more than one episode. He is frequently a comic foil to most of the main characters, including T.J. He also gives good moral advice to all of his children (and sometimes even to Mo).

Morris L. "Mo" Tibbs

Played by Omar Gooding, the character of Mo Tibbs is Marcus's best friend. He is typically portrayed in the show as being dimwitted (such as confusing a rasher with a haberdasher) with some signs of gullibility, to the point where in the episode "I Was a Teenage Sports Wife", T.J. makes Mo part of a placebo experiment by giving him sugar-filled pill capsules that T.J. claims will stimulate synapses in the brain (therefore increasing intelligence), in order to better perform on tests, only for Mo to inform other students about the pills and drowning T.J. with offers for the "brain pills". Despite his heavy and muscular build, Mo is usually depicted as a friendly person (the pilot episode, however, depicts him as being almost a rival to Marcus in a few scenes, particularly one in which he gets into a fight with Marcus over their classmate Mariah (Venus DeMilo Thomas), whom they both are romantically interested in, resulting in T.J. stepping into the altercation to defend Marcus). Mo is discovered to be an excellent chef, due to the tutelage of Yvette (who helped him learn how to cook after Mo signed up for a cooking class on the spur of the moment, simply because the class was cooking bacon at the time), he plays bass guitar in Marcus' band Mackadocious, and is also a semi-talented barber.

Floyd is often annoyed with Mo's antics, such as his constantly eating from their fridge and even once having slept in T.J.'s bed (having done so as part of a plan to help T.J. sneak out to see a friend at an arcade after being reprimanded and effectively, grounded by Floyd over his choice of clothing). Despite this, Floyd seems to genuinely care about Mo and, as seen in the episode "Diary of a Mad Schoolgirl", shares with him a passion for a barbecue. Mo all but lives at the Henderson's house and dislikes eating at his own house (having once chipped a tooth while eating oatmeal cooked by his mother, as he notes in the opening scene of "Get a Job"). In the season three episode "That's My Momma", Mo accidentally overhears a conversation between his parents Delroy and Verla Mae that they had adopted him as a baby, this leads him to have a falling out with his parents and Marcus and T.J. helping him find his birth mother who is revealed to be a fortune teller in Delaware named Shelia Ecks. A recurring catchphrase of his throughout the series is, "Hello der!" ("hello there").

The Henderson house

A majority of the show's scenes take place in the Henderson house or the school. Originally, most of the family scenes took place in the kitchen, with the lack of living room area seen during the first season. The living room set when it was added in season two had a window and the front door at stage center, and the stairs and kitchen doorway at stage right. The kitchen set was scaled back as well with the back stairs that lead to the bedrooms removed. The layout changed again in season three with the kitchen remaining the same, but the living room layout changes including the front door now located at stage left. Marcus and T.J.'s bedroom is the only set to remain the same during the course of the entire series.

Piedmont High School

Piedmont High is the high school that Yvette, Marcus, Mo and T.J. attend. The four main sets seen in the show are the classroom, the hallway, the cafeteria and the auditorium. The hallway changes its layout twice during the series.