Who is Melanie from Sag Harbor and what is their importance?

Melanie visits Sag Harbor the summer of 1985 to spend a few weeks vacationing. She remembers Benji as the boy, on whom, she once had a crush. Despite dating Nick, Melanie often reminds Benji of their past. One night Benji and Melanie kiss, but then Melanie refuses to acknowledge that anything happened between them.