Joe = Lonely, archtypical nerd from a family with ridiculously high expectations for him. As he gets older, they expect him to follow in both of their footsteps, as well as his brother's - all successful doctors.
Matt = Child of divorce, is at the stage in life where he understands that there must be an alpha-male of the group, and vies as much as he can for that position. Also, with his younger brother in tow, he is burdened with extra responsibility.
Mimi = Materialistic, ditsy, has probably grown up on the idea that a prince will wisk her away to domestic housewifedom where she can live happily ever after. Is probably just discovering sexuality, and is not only afraid of it, but has had her ideal, fairy tale-like ambitions darkened upon finding out what "growing up" and getting married, starting a family, etc. actually involves.
Tai - Threatened by Matt's presence, has also just begun to discover sexuality. Tries as he might to understand what is going on with him and those around him, but always seems off.
Sora - A girl who has seen herself as "one of the boys" for most of her life, and is just starting to realize her femininity, which she is obviously threatened by.
Izzy - A genius from a very young age, probably feels that most people expect too much for him. Desires to be a child for just a little longer.
T.K. - Child of divorce, whose situation has caused him to grow up earlier than he'd like to - has lost two major adult influences, his father and his older brother.