Character Analysis
Sarah is Lennie's best friend, and the first thing Lennie tells us about her is that she's full of contradictions. Specifically:
Sarah is the most enthusiastic cynical person on the planet […] she started wearing black (even at the beach), smoking cigarettes (even though she looks like the healthiest girl you've ever seen), and obsessing about her existential crisis (even as she partied into all hours of the night). (2.25)
Hey, coming from a family like the Walkers, Lennie would never settle for an ordinary best friend. But Nelson pushes Sarah past the "quirky best friend" stock character by making her genuinely thoughtful. Sarah's a reader, and like Lennie, she constantly has a book in her hand; after Bailey died, she read several books on grief.
As you might've guessed from the reading-books-about-grief bit, Sarah's a really good best friend. When Lennie texts her after avoiding her for weeks, Sarah calls her right away to make hangout plans. Sure, Sarah struggles with forgiving Lennie, since she clearly misses and feels abandoned by her, but she's there for her anyway, whether that involves showing up at Lennie's work to commiserate or brandishing a flask when Lennie sees Joe with another girl. It's thanks to Sarah's effort that Lennie becomes a little less antisocial and tries harder to be a good friend, checking back into her life as she does so.