Nisekoi: False Love Vol. 4 (manga)

It's hate at first sight when Raku Ichijo first meets Chitoge Kirisaki. But much to their chagrin, the two are forced into a false love relationship to keep the peace between their feuding gangster families. Meanwhile, Raku's still hung up on the girl he made a promise to ten years ago, keeping a pendant around his neck as a memento. The girl, in turn, holds the key that opens the pendant. To Raku's surprise, he discovers two girls have keys from a promise they also make ten years ago, Chitoge and his current crush Onodera! Making matters worse, another girl with a key appears on the scene..