While Glowing Fish makes an excellent point, he has forgotten to mention some of the other female personality types featured in the Sweet Valley Twins books, who are also basic:
Enid: Elizabeth's best friend and companion in martyrdom. Enid is always described as being "nice" and "smart"--Sweet Valley euphemisms for "boring" and "unpopular." Enid's only real asset is her friendship with Elizabeth.
Lila: Jessica's best friend. She and Jessica are essentially the same person, although Lila is a brunette. (In Sweet Valley, this is apparently more alien than being from a different planet.) Jessica is often portrayed as learning a lesson or demonstrating compassion, whereas Lila is completely heartless. Needless to say, Lila is the more realistic of the two.
Many women who say they are Elizabeths are actually Enids who secretly want to be Jessicas.