Our dear protagonist, Mclean Sweet, is a teenager going through a crisis of identity—among other things. She's just moved to the town of Lakeview with her dad, and is trying to finish out her last year of high school with as little drama as possible.
As she's moved from school to school to accommodate her dad's high-travel consulting job, Mclean has gotten really good at creating new personas for herself. Whenever she goes to a new town, she picks a new name and a new identity:
It was so easy. Just like that, in the hurried moments between announcements, I wrapped up and put away sixteen years of my life and was born again, all before first period even began. (1.28)
You'd think that she was on the run from the FBI or something, but nope—Mclean just finds it easier to play a part whenever she goes to a new school. It pretty much guarantees that she won't form close connections with anyone, which is important to her since she'll just up and move again before too long.
Ever since her parents' nasty divorce, Mclean hasn't been able to live life like a normal teenager. She's taken her dad's side in every way, choosing to see and talk to her mom as little as possible (which drives her mom absolutely nuts). She's also taken it upon herself to practically parent her dad; she makes sure their home is in order and that he's not lonely or sad:
[…] I knew there were so many things about his life that I couldn't fix, no matter how hard I tried. It was probably why I worked so hard to handle the things I did. Getting us settled, taking care of details, keeping the chaos we'd chosen as neat as possible. (2.68)
It's pretty obvious that Mclean has taken on some very adult responsibilities since her parents' divorce, and she doesn't complain about it. In her mind, it's the least she can do for her dad after her mom screwed him over royally.
Speaking of Mclean's mom… Mclean definitely knows how to hold a long-standing grudge when it comes to her betrayal. Even though it's been three years and her mom makes every effort to stay close to her, Mclean just isn't having it:
[…] I was suddenly infuriated. My mother was still talking—God, she was always talking—as I stomped to the open door and out onto the deck. (6.42)
She's not interested in hanging out with her mom because it feels like a direct betrayal of her dad—even when he assures her otherwise. It's only when Mclean begrudgingly agrees to start talking to her mom again that she feels like she can slowly let go of all her anger… and maybe rebuild a relationship with her mom. It won't be the same as it was when she was a kid, but it'll at least be a functional relationship—and Mclean won't have to carry so much hurt around all the time anymore too.
Following the crash and burn of her parents' beautiful marriage, Mclean is pretty skeptical of any relationship ever working out. She says:
In truth, since my parents' split, I hadn't had much faith in relationships and even less of an inclination to start any lasting ones of my own. (2.92)
Even though she has a huge crush on Dave and he gives her all sorts of butterflies, she keeps her emotional distance because she just doesn't see the point. Isn't every relationship destined to fail in an epic torrent of tears and betrayal? Girl has got some major hang-ups when it comes to connecting to people after her mom cheated on her dad, destroying Mclean's faith in true partnerships.
And it's not just romantic relationships that Mclean has a problem with—she's pretty guarded when to friendships as well. But the residents of Lakeview are pretty persistent, and soon she finds her walls cracking. She ends up with some friends and a boyfriend, and it's not nearly as bad as she thought it was going to be—not even when it comes to her mom. Go figure.