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If you remember, when the Whitebeard war was over and Luffy invaded Marine HQ again along with Jinbe and Rayleigh Buggy was seen speaking with Alvida about it. When he learned that Rayleigh was with Luffy he said "Luffy's father is Dragon and his grandfather is Garp, so Rayleigh is his uncle". Well I believe in this theory as Buggy has spent a lot of time with Rayeigh and knows some information that we don't. But I don't think it's his uncle, but his great uncle as he and Garp are about the same age, so they could be brothers. Or maybe he's Luffy's grand mother's brother, who is not revealed yet