From TVTropes.org: "Many years ago, Alice and Bob met each other by random chance, and the meeting left a deep impression on at least one of them. But years have passed, and now they're grown up (or at least in High School). All unknowing, they have been pulled back together by the Red String of Fate - friendship, romance and/or adventure will see them rekindle a long-lost connection long before they realize that they've met before. The realization that their new friend is the same kid they met years ago may come slowly with heavy foreshadowing, or be a sudden twist."
Warning: spoilers ahead!
Warning: spoilers ahead!