Lindsey, Susie's little sister, is thirteen when Susie dies. She is the living hero of the story Susie is watching and telling. Her story begins in winter in chaos, and ends in spring in reunion, marriage, and even a cute little bambino. She doesn't seem to have any bad qualities or even flaws, except, perhaps, being a tad too hard on Abigail. (But who can blame her?)
Although Susie is smart, Lindsey is considered a genius. She's not only a brainiac, but also one of the most physically fit people we know, able to run eight miles in the rain and leap from a killer's window in a single bound. Susie gets so caught up in Lindsey's life, especially the romantic aspect, that she almost thinks she's Lindsey sometimes. Late in the novel, Susie says, "I collected my college diploma and jumped on the back of Samuel's bike […]" (17.1). But, then she clarifies, snapping out of her fantasy:
Okay, it was Lindsey. I realized that. But in watching her I found I could get lost more than with anyone else. (17.2)
Through Lindsey, Susie can live her Earthly fantasies and eventually gain enough satisfaction to move on to deeper, more heavenly desires.
Yes, Lindsey stars in two very different scenes of running. In one, she's running for her life, away from Susie's killer. In the other, she's running freely, abandoning herself to the romance and joy of her life. In both scenes, Lindsey is running toward the same thing: home. In both scenes, she has the same ultimate goal: getting home safely to give Jack Salmon, in particular, the good news. Lindsey's relationship with her father is extremely touching.
After Susie's death, she becomes increasingly protective of Jack, and she even switches places with him, metaphorically, in the critical break-in scene. She knows that Jack is defeated in his attempts to prove Harvey's guilt, and she steps in to do the job for him. The break-in is also for Susie. Lindsey's being in Harvey's house reveals his other victims to her. Susie needs to know what he's done, in order to be able to be in commune with the other girls and, eventually, to move on.
Lindsey also turns some stereotypes on their heads. Like Abigail, Lindsey finds comfort in consensual, romantic sex, the opposite of what Susie experiences. Like Abigail's affair, Lindsey sexual relationship with Samuel could be considered wrong. She is only fourteen, after all, when she begins having sex. But, Lindsey and Samuel seem to have handled it responsibly. Through Lindsey (in part), Susie's finds beautiful contrast to the brutality of her sexual experience with Mr. Harvey.