Abbigale is James' love interest in the book.
Abbi goes through an excessive amount of hardships and tragedies including being gang raped by her friends, having her unborn child beat out of her by her psychically abusive boyfriend Seth, she was abandoned by her mother, she lives with an abusive alcoholic father, and her trusted guidance counselor turning her back on her after her miscarriage. This all takes place before the book begins.
Although Greg doesn't describe Abbigale's appearance in the book, James describes her as beautiful many times.
Abbigale vs Real Life
On April 2, 2015, Greg mentions his wife, Taylor, does not like the character Abbi. (Soruce) Some assume this is because he based Abbi off of a real girlfriend he had. Many people speculated if she was based on his teenage love, Tanya.
On April 14, Greg revealed he based the character Abbigale off of a girl he knew named Abbi. Greg says that the Abbi he knew in real life did have an abusive boyfriend, was a cutter, and he wrote her the letter that James sends to Abbi in STA. (Source)