Which promised girl is Raku referring to? Onodera, Chitoge, and Marika seem to own a key and all of them have a fuzzy memory of their childhood. I think Raku really met them all when he was young and made a promise to each, maybe, so that makes all the three promised girls. Of course, only one key can fit inside Raku's locket but it's not whose key can fit but the girl he made a promise to. So the point of the story is whether Raku chooses the "promised girl" or his present love? Or I think is should be which of the three promised girls is his true love? So the one Raku's referring to as the "promise girl" is which: the girl with the key to the locket or the girl whom he promised to love?

Sorry I think my question sounds a bit confusing but I hope you try to understand it.
As the manga has ended, we have a winner. And the promised girl is

(click to enlarge picture, pictures taken from chapter 221)


   Chitoge





   


But wait, there is more


   Chitoge overheard that Raku actually like Onodera





   





   So Chitoge gave the key to Onodera





   


So, in the end, the promised girl is


   Onodera





    


So to answer your question, the girl with the key to the locket is the girl whom he promised to love