When we first hear of Hilary Handy, she's a potential suspect for Amy's disappearance. According to Amy's parents, she was kind of an Amazing Amy groupie back in high school—she was not only Amy's best friend, but dyed her hair the same color as hers and dressed like her. This was kind of cute, but then Hilary started making disturbing phone calls to Amy's parents about how she was the new Amy and wanted to come live with them. Whoa—sounds like the plot of a teen psychological horror film.
Here's the problem though: That story's totally fake. Or, the events actually happened, but the motives behind them were different. In reality, Amy coerced Hilary into doing the dye job and mental manipulation of her parents to set Hilary up, and when Amy pulled the ultimate act—throwing herself down the stairs, breaking her own ribs, and claiming Hilary pushed her—Hilary looked "totally insane" (39.28) and guilty. It was an elaborate act of bullying on Amy's part, but Hilary ended up leaving school anyway instead of continuing to navigate life with Amy.
These days Hilary's a pretty ordinary person, married with three kids. But that doesn't mean she doesn't play an important part in our book's plot.
What Hilary's character ultimately contributes to Gone Girl is showing the extent Amy will go to exact her revenge. Hilary's offenses amount to a few occasions where she was chosen over or achieved more than Amy—barely enough to cause a catfight between two teenage girls, let alone a full-frontal psychological attack. And yet that's exactly what Amy does to her.