Nisekoi: False Love Vol. 1 (manga)

It’s hate at first sight—or rather a knee to the head at first sight —when Raku Ichijo meets Chitoge Kirisaki! Unfortunately, Raku’s gangster father arranges a false love match between Raku and their rival gang leader’s daughter, who just so happens to be Chitoge! Raku’s searching for his childhood sweetheart from ten years ago, however, with a pendant around his neck as a memento...but he can't even remember the girl’s name or face!