Episode 21 and it's not the Lost Masters arc. It's the Lost Master arc. Singular. No S. And once you notice that, the whole question changes. In this episode I break the phrase down into its parts: what "lost" actually requires — you can't lose something you never had, so this is someone we've already met; why deaths in Union Cross might not count when Daybreak Town was a simulation restarted over and over to draw something out; why the Keyblade Graveyard having its name before the first Keyblade War we ever see is a chicken-and-egg problem the Hollow Bastion precedent makes worse; and why Ava — the obvious answer, the Master even Luxu shrugs about — has a form-mimicking ability that means she may not be absent at all. Then it gets philosophical: reformed selves aren't the same people, you don't step in the same river twice, "lost" is a moral dec ...