Joy boy was the holder of the giant straw hat, so he would be a giant. That would explain how he lived at least 100 years of great emptiness. Joy boy was such a man who had as his goal what everyone in the world could live in peace and harmony, without discrimination and marked social differences. For this he stole treasures from the ancestors of Im sama, probably reigning, and hid on Laugh Tale the huge treasure, with even the poiggne griffe with his exploits written on it that, incidentally, made Gold Roger and crew laugh when they read the poiggne griffe.

Joy boy, however, failed in his attempt at unification, for example he failed to bring to the surface the fish men to whom he left a poiggne griffe with an apology engraved, but said that one day someone would succeed, namely a owner of the D. One was supposed to be Gold Roger, while the next should be Rubber. The dream they would have in common is to be free, not only them but everyone, free to follow their dreams and goals. The One piece would then be the stolen treasure, the poiggne griffe, the All Blue that would result from the destruction of the Red Line that divides the world into the 4 seas at the hands of ancestral weapons

And finally, the D was chosen by the author simply but brilliantly for the smile :D although it will definitely have real meaning.