Their mother died when Katara was 5, Sokka 7, so they lived alone with their father for most of their lives, then their father was sent to the war and they now live with their Grandma (Gran-Gran). Maybe that's why Katara is a little tomboy-ish, no mom and all... she only has male friends (Aang, Haru, Sokka, too) and Toph, too... but she is not a very girly girl XD  Katara is in the swimming team.
Sokka is your average big brother, doesn't like boys around Katara but doesn't see any threats in Aang or Haru (because... they are Aang and Haru lol), he is the captain of the Middle School basketball team (or something fancy like that lol) and has more girlfriends than he can count! He is the class clown.
Zuko's and Katara's parents knew each, so they know each other since they were babies. They actually get along but fight A LOT, they always have…
Why Katara likes Zuko? She likes him for him, not his looks or money like the rest of the school. (They have known each other for a long time)

Zuko and Azula live with their parents, Ozai is the CEO of some well-known company, and Ursa is a stay-home mom. Iroh lives in another state and his son was killed the war.
Zuko was born really sick (lucky to be born, remember?) so he is kinda small for his age, and really overprotected by his mother.  Zuko may be the reason Ozai and Ursa got married in the first place...
Azula is pampered by her father, really spoiled, very precocious (flirts with *everybody*, Sokka, Jet, Haru, etc). She loves to tease Zuzu.
Jet stalks Zuko XD because he lets him play with his PSP (Zuko is sweet like that), and he has a lot of expensive stuff. Zuko is a quiet asian videogaming semi-nerd, keeps to himself; few friends, avoids girls in general,  still HAWT tho, and doesn't realize lots of girls like him. He's tragically addicted to Twinkies (Thanks, Akavertigo! XD)
Azula has Mai and Ty Lee, of course, they are about the same: Rich, pampered, popular girls. Tylee crushes on Sokka and Mai likes Zuko.

Aang is an orphan adopted recently by Gyatso. He is hyper, super cute and innocent. Huge crush on Katara and hates Zuko with a passion.

Toph goes to a private school (she is very rich and overprotected. And her parents force her to wear dresses) so she sneak outs and plays with them in the neighborhood. In this AU she is not blind, but suffered from temporary blindess when she was little which caused her parents to overprotect her.

Son of a single mom, he enjoys sports and running around the neighborhood following Katara’s orders.

His parents didn’t have much time for him, so he is a bit of a problem child (he likes pranks, skipping school and pretty much doing nothing all day). He likes stalking Zuko and “borrowing” Zuko’s PSP (and others expensive ‘toys’-ipod, laptop, etc.). He finds Katara cute. Jet and Katara date in college.

Jin: Zuko's college girlfriend.