As a child, Sam was unpopular and teased because her skin had a tendency to turn "red and splotchy" when she was embarrassed. When Lindsay recruited Sam to help bully her rival, Beth Schiff, at the end of elementary school, Sam suddenly became popular and has been ever since. She went from being bullied to being a bully. She does not see herself as a bad person because she justifies her behavior as simply trying to "stay on the laughing side." When she dies in a car accident, the universe puts her in a kind of purgatory, making her relive the day of her death until she redeems herself and is able to move on.
While tough on the surface, Lindsay's armor is protecting some major dysfunction. The first trauma was her parents' divorce, which led to Lindsay's bed-wetting. To protect herself, Lindsay blames the bed-wetting on her best friend, Juliet Sykes, and continues to bully her for years. The second trauma was a case of statutory rape. Lindsay is also bulimic. To avoid facing her secrets, Lindsay takes out her bad feelings on others and enlists insecure girls, like Sam, to be her minions. Lindsay is cold, calculating, capricious, and moody. But even though Lindsay's instinct is to be cruel, Sam claims to respect her and is sure she will figure out how to be a better person one day.
Kent and Sam used to be friends in elementary school. Kent even kissed Sam in third grade. However, because he wears odd clothes, including a bowler hat, to school, Sam considers him too much of freak to date, even though she thinks he is handsome. Kent becomes an instrumental part of Sam's learning to embrace goodness and her authentic self. He reminds her she was once his hero, coming to his aid when he was bullied in second grade. Ever since, he has wanted to return the favor and be Sam's hero. Over the course of the narrative, Sam comes to realize Kent is the person she should be with, and they share her first real kiss.
Despite her quiet beauty, Juliet is unpopular as a result of Lindsay's vendetta against her. Once best friends, Juliet helped support Lindsay through her parents' divorce. But when they shared a tent at camp and Lindsay urinated in her sleeping bag, Lindsay blamed Juliet for doing it, giving her the nickname "Mellow Yellow." Lindsay continued her bullying because Juliet never stood up for herself. At first, Juliet felt compassion toward Lindsay and thought the situation would improve with time. but it didn't. Juliet now feels broken and no longer has the hope she needs to stay alive. She dies by suicide on all of the days except the last.
Rob has always been popular and once claimed in sixth grade that Sam was "too big of a dork" for him to date. He is overly concerned about his social status and overly aware that people are always watching him. Sam notices he keeps his eyes open when he kisses her. She realizes she likes the idea of him, but she doesn't actually want to date him. By Day 7, Sam has worked up the courage to break up with Rob. He is angered that she would do such a thing. Sam believes he will make a sorority girl very happy one day.
Elody became Lindsay's friend in ninth grade after hooking up with Lindsay's crush at the time. The other girls call her a tramp because she has had sex with several boys, likely at Lindsay's behest. It is not clear why Lindsay chose to befriend Elody rather than bully her, but Sam thinks Elody is "the nicest of any of us." Elody's family is not rich, she doesn't have access to a suitable car, and her mother often comes home drunk.
Ally became friends with Lindsay through field hockey. Her father is a lawyer for the music industry, and she lives in a big house, bigger than the other girls' houses. Ally is passionate about cooking, although she eats little of what she prepares. She seems to be the peacemaker among the friends.