Jessica and Elizabeth Wakefield
There's a page or two long description of them at the beginning of every book, pounding it into our heads every time that the twins have "perfect size six figures", "flashing blue-green eyes" and "perfect skin". Basically, they are the most beautiful girls ever created and we should be glad Sweet Valley God made two of them.
Jessica is flashy, flirty, and jumps on seriously almost every guy that looks her way (and even those that try to get away). She is manipulative and can get her doormat twin Liz to give in to her demands almost any time. She schemes to break couples up and bring them back together. (A good example of this is how she brings Steven and her friend Cara together, and then tries to break them up twice because they're boring her.Yeah)
Elizabeth is the loyal, caring, angelic twin, dating steady boyfriend Todd Wilkins, (that is, until Todd moves away), and is best friends with quiet Enid Rollins, who Jess can't stand. She writes for the school paper, (the Oracle), and has a "secret dream" to become a writer. It's not really that secret since everyone seems to know.
Liz is always bailing Jessica out of the crazy things she gets herself into. Just a few of those things? Kidnapping, Cults, bad boyfriends, sleazy older men, shipwreck, and stalkers. And that's really just a few. Elizabeth does get into her own scrapes occasionally, like the time she came out of a coma and started acting like Jessica.
Or the whole evil-twin-car-accident-miniseries thing that comes up later in the series.
Elizabeth isn't satisfied to just save Jessica from her scrapes, she also meddles in everyone else's affairs. Anyone that meets her confesses their deepest secrets to Liz, even if they barely know her. She's the gossip columnist of the paper, so how she got the rep of being the girl that keeps secrets, I'm not totally sure.
Jessica, on the other hand , is mostly a total sociopath that ruins lives, or at least tries to. She can't stand anyone getting in her way, whether she's going after a guy, (which happens at least 1000 times a book), a trophy, or a title. (She really proves this when she spikes her own twin's drink just to beat her out for Prom Queen).