Sarah's had a crush on Ryan since at least the 8th grade, something she thought her best friend Brianna knew about. But Brianna starts dating him anyway, determined Ryan is the one for her. Small problem--Sarah still has a crush on him and Ryan appears to have a crush on her.
The Unwritten Rule--don't fall for, don't date, definitely don't kiss your best friend's boyfriend makes Sarah feel especially guilty. Brianna has a hard home life--her parents either ignore her or treat her like a complete failure, which means Brianna needs support from Sarah and Ryan. When Brianna makes out with another guy to get Ryan's attention and it doesn't work, Sarah may think she's found a way to calm her own conscience.
Sarah tells a good story. She is the friend that is "stealing" the boyfriend, but you don't hate her. Brianna gives her a lot of backhanded compliments and is angry and perplexed why Ryan would like Sarah more than her in the first place. Sarah and Brianna's friendship doesn't survive, but as the reader, I didn't really want it to anyway. I found Sarah to be a much more likable character. She shows remorse and Brianna never does.