Greg says he wrote Abbi the same confession/love letter James wrote to the Abbigale character in his book. Greg says Abbi was a cutter and had a verbally abusive boyfriend, similar to his character. (Source)
Oddly, Greg never wrote about Abbi on any of his past websites. Suspiciously, this includes his 2005 "Digging Deeper" Onision.com page, where he listed important people he had known, or his 2005 "A Life's Story" page, where he listed important relationships he had. (Source)
It is possible Greg lied and she was named Abbi so that people would not look for her on his past websites, where he named people by full name. It is also possible Greg just never wrote about Abbi on his sites. He did state if he forgot someone's last name, he skipped writing about them on the "Digging Deeper" page. Although, this would not explain her absence from his "A Life's Story" page.