Zoe and Ryan had known each other their whole lives. They grew up on the same street in a tiny town where everybody knew everybody. As little kids, they spent their summers running through each other's backyards, catching fireflies in jars, and sharing peanut butter sandwiches at lunch. They were always together. Where one was, the other wasn't far behind. Their moms used to joke that they came as a pair, like shoes.

High school was when things started to feel different, though neither of them talked about it out loud. Zoe started growing into herself — her hips getting rounder, her laugh getting softer somehow, more aware. Ryan shot...