Sabo's existence in One Piece is revealed during Marineford ... Oda shares a hypothetical scenario where Sabo saves Ace at Marineford, but it's considered a dream by Luffy and Sabo.

The latest chapter of One Piece ... fully reveal Luffy’s real dream at long last, if not tease it even further. So far, the only people privy to Luffy’s true dream are Ace, Sabo, Yamato ...