Abby is well known for being very nasty to people. She is adequately popular in high school and flirts with a lot of the football players, whom she seems to be friendly with. In reality, she is probably a very lonely person, who has a nice, normal family in one of the richer families in Capeside. As it turns out, although their relationship isn't exactly healthy, Jen is probably the only real friend Abby ever makes in Capeside.
When we first meet Abby in Detention, she already has a reputation for being horrid among the gang (minus Jen, who has never met her before). She comes off as a kinda bored, instigating teen who revels in toying with other people's lives.
This perception of her is only brieftly altered with Jen in the periods where she spends time with Abby (which usually coincide with Jen feeling left out of the gang).