The title character. Once a street-wise youth in Philadelphia, a skirmish with a bully causes Will to be sent to live with his wealthy relatives in Bel-Air, California. The show focuses on Will living and interacting with the upper-class society and residential celebrities. Later on, there's more focus on Will's college life.
Butt Monkey: At times. Notably, he's been knocked and humiliated a few times by more intimidating guys.
The Casanova/Lovable Sex Maniac: Quite obsessed with scoring women. One episode shows his list of "college activities" only have sororities on it. As Will puts it: "From Alpha to Gamma to Bad Mamma Jamma!"
Character Development: Primarily regarding his relationship with Carlton. Early on, Will gives Carlton grief about not being black enough, but eases off once he sees Carlton face discrimination from other blacks for exactly this and realizes how asinine and unfair it is.
Will was pretty much sought to retain his free-will "Homeboy" lifestyle when first settling in Bel-Air, but has become more accustomed to it and more open about his knowledge of numerous references.
Will:(singing and dancing weakly) I got no more fever, I got no more fever ... Aunt Viv?
Will: Catch me. (faints)
Disappeared Dad: Will's father Lou took off on the family when he was a baby and left his mother to raise him. When Lou comes back, he briefly tries to reconcile with Will but proves he's still the same selfish bastard he always was when he effectively abandons Will once again. This led to one of the series' biggest Tear Jerkers when he tearfully asked Uncle Phil:
I Have No Son: Inverted with Will and his father Lou. When Lou comes back into Will's life after having run out on him 14 years ago, Lou then abandons him again for another trucking job. This is the final exchange Will and Lou share:
Wouldn't Hit a Girl: Which leads to a humiliating defeat at the hands of a female boxing trainer.
Phillip Banks (James Avery)
A successful attorney (later appointed a court judge) who doesn't try to hide his reluctance to Will settling in with the Banks. Spends a good portion of the show dealing with Will's antics and help fix any mess made.
Best Served Cold: Normally Phillip would yell or threaten Will the instant he does something insanely wrong. But in one case Will did something so bad, that it took three years for Phil to think up the ultimate punishment for Will. Will really had it coming, too - he wanted to sleep with a girl who wouldn't give it up before marriage, so he staged a fake wedding with Jazz playing the preacher, just to get her into bed. He came to his senses at the last second, but damn.
Bumbling Dad: Averted. He has his moments of stupidity, but he's portrayed as competent and knowledgeable way more often than other sitcom dads.
Doom It Yourself: Phil is an excellent lawyer and an even better judge, but his skills as a handyman are less than impressive. His attempt at fixing the stove burns Geoffrey's eyebrows off, his attempt at fixing the toaster causes a piece of toast to fly into Geoffrey's eye, his attempt at fixing the static on the phone completely kills the line, and his attempt at fixing the sink causes a leak. At one point, Vivian has to use a Lysistrata Gambit to stop him from doing any more damage to the house.
Self-Made Man: Was born a country boy and works himself up in his career to achieve his current wealth.
Stern Teacher: This is his general parenting style in dealing with Will and Carlton. He loves his boys very much, although he doesn't hesitate to put the fear of God into them with his Scary Black Man routine to keep them on the straight and narrow.
The Other Darrin: She was originally played by Janet Hubert-Whitten. From the fourth season on, she was replaced by Daphne Maxwell Reid (there was even a lampshade about the fact that Vivian's skin color change, as the original actress had dark brown skin while her replacement was a light-skinned black woman).
The middle child of the Banks family who strives to get into Princeton and be a successful lawyer like his dad. More confident in the law and order in general, Carlton's at odds with Will more than frequently, though by the later half of the series he becomes The Lancer.
Angst Coma: When Carlton fails to get into Princeton.
Beware the Silly Ones: His short stature and overall behavior aside, Carlton actually managed to knock out Will in more than one occasion, whether it be by one punch, or even a blow from his elbow. He once even let Will take a fall from a treehouse by releasing the ladder after Will had started descending it, this after having taken yet another dig at Carlton's height.
Took a Level in Badass: When he started out, most of the time he relied on either Will or Phil to back him up. However during an episode where someone accuses him of "Not being black enough" he drops the goofy nerd persona and proceeds to verbally tear the Jerkass a new one.
The spoiled elder child of the Banks family, Hilary's socialite bent on looked highly upon her peers in the upper class. She's also leeches off her parents' fortune and ignorant of how life outside hers really works.
Character Development: Originally shallow and lacking self-confidence for being a jobless college dropout, she is shown to be more and more jovial for each successful job she takes on from Season 2 onwards; she ends up starring in her own hot talk show and moves to New York.
Womanchild: Hilary is hopelessly immature and can't be self-sufficient, so her parents tolerate her freeloading.
Ashley Banks (Tatyana M. Ali)
The youngest child of the Banks (until Baby Nicky arrived), she very much looked up to Will. Gradually, she became bent on being less of daddy's little girl and more open and a bit rebellious, even out of Will's control. She is the only one of the Banks children to have, more or less, normal adolescence; having common teenager issues.
Bratty Teenage Daughter: Averted, except in one case where she had a singing career and got a big head, but when her album didn't sell, she went back to being her sweet self.
Characterization Marches On: Originally very respectful and gentlemanly (in season 1), he becomes more and more a cynical Deadpan Snarker who openly hates his job and the Banks family, to the point that literally everything to come out of his mouth is a sarcastic remark or an outright insult (towards everyone except Vivian, Ashley, and Nicky).
Chronic Villainy: Geoffrey switches back and forward between plotting against and hating the family and feeling genuine affection for them throughout the series.
Chekhov's Shirt: Since they constantly reused the same clip of Jazz getting thrown out of the house, they maintained continuity by having Jazz wear the same shirt in almost every episode he gets thrown out. It gets to the point where you can tell the Running Gag is about to occur if you see him wearing the shirt.
Running Gag: His tendency to get thrown out of the Bankses' house.
Inverted, subverted, and averted a couple of times, such as when he was thrown into the house, when Jazz threw Uncle Phil out the house, and when Jazz did something so horrible that Will doesn't think he's worth kicking out of the house. Also invoked by Jazz himself to show that he can reach the car from the house without walking.
Hidden Depths: At first he seems like sort of a hood with no real interest in school. Will later reveals that without Trey, he never would've made it back in Phillidelphia, as he protected Will from bullies trying to jump him for his books.
It's All About Me: If someone pays a compliment to somebody in his vicinity, he takes it for himself. Additionally, he was more insulted by Jazz not knowing who he was than by Jazz making advances on Hilary.
Jerkass: Judge Robertson was apparently idealistic in his younger days, but as of the show's timeline he's become extremely selfish, misogynistic, deceitful, and a classic example of a corrupt politician. He was such an awful human being that at his funeral, everyone was there to celebrate his death, and they even applaud Will when they learn that he caused it.
Unrequited Love Switcheroo: It's implied that Jackie had a lingering crush on Will before he left and that he hadn't quite realized his feelings until he was gone, thus explaining why he never called because he missed her too much and didn't want to come to terms with his feelings. When she comes out to Bel-Air, Will starts to fall for her again, though the situation suffers from Aborted Arc and is never resolved.