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Segreto
Marco knew he was the envy of many pirates, and quite a few marines as well. Commander of the First Division of the Whitebeard Pirates, holder of the Tori Tori no Mi, Model: Cryophoenix, one of the rare mythical zoan fruits. A man whose name was known throughout the whole world.
The pirate would never admit that when he ate the thing he had been well on his way to starving to death. His pride would never recover from the blow. Upon arriving in this world, the first local Marco had found himself in hadn't been very hospitable to say the least. It put the desert in Desert Island.
In truth, the young man had found the oddly colored blue fruit several days before consuming it, setting it aside for when he got truly desperate. Rightly suspecting there was something not quite normal about the strange fruit. In that, Marco had been right. Fortunately however his first hypothesis, that it was poisonous, hadn't been. The Devil Fruit had undoubtedly saved his life. Keeping the then teenager in the peak of health while he learned the limits of his new power.
Marco's first flight made under his own power had been awkward to say the least. Fortunately, perseverance was one trait the pirate possessed in both his lives. And the far friendlier looking island right on the edge of the horizon was a great motivator. The Devil's Fruit had even fixed his eyesight, though the marks of his glasses still existed in his phoenix form. As Marco aged, the sun darkening his skin and bleaching his hair a light orange, those marks became the only sign Harry Potter had once existed.
And truth be told, the pirate was glad of it.