I take the easy way out and link to the fhwiki entries.
Buffy Summers. Cliffnotes: She's tiny, enthusiastic about cheerleading and sharp weaponry, doesn't yet know Angel or much of her future (except that at some point she'll get a sister - she thinks Dawn is adopted) and is spending the second summer term in Sunnydale which she'll be seeing for the first time.
She is from Buffy the Vampire Slayer. Guess which one she is.
Important note for cabin-mates: She's bringing
Berthold with her, but he'll mostly stay under her bed so they needn't even know he's there unless they want to.
Emma Frost. Cliffnotes: She wears white, it's probably skimpy and shows off her boobs, she's very much in love with Scott Summers, but that doesn't keep her from flirting and she's a telepathic living diamond.
She is from the world of the X-Men.
Sabriel. Cliffnotes: She's just arrived! She's a reverse Necromancer; she puts dead things back to death instead of bringing them back. She's determined, unused to being around boys and responsible. She's also kind of impatient and doesn't like it when people pretend to be something they're not. She is a magic user.
She is from the Abhorsen trilogy, which you should so read.
Questions? Comments? Coffee?