Nisekoi. False Love. Volume 2: Naoshi Komi

Though Raku and Chitoge's false love is off to a rocky start, they have everyone fooled! However, Raku's still hung up on his childhood sweetheart and wears a pendant around his neck as a memento. But one big problem prevents him from finding the girl whom he promised to marry - he can't remember her name or face! To further complicate matters, Onodera, Raku's current crush who also harbors secret feelings for him, accidentally overhears the two arguing over their false relationship!