This is just one of those things that you think is not likely to happen, but it happens. As it turns out, Z-one (pronounced "zone"), who has been spoken of as the main antagonist of Season 2 and the leader of Yliaster himself is none other than Yusei Fudo himself, the main protagonist of the series. This isn't just some dark side of a previous hero, such as Yami Yugi and the Supreme King. This is the real deal Yusei Fudo from a ruined future gone back in time to change things around, which in turn links to the goal of Paradox, the villain from Yu-Gi-Oh! 3D: Bonds Beyond Time. Sad to say, this hero of 5D's, whom has shone high in New Domino City and that so many looked up to is now officially confirmed to have formed the dangerous Yliaster, meaning he is the one who jerked everyone's strings from the very beginning, including Roman's fall and the formation of the Dark Signers. You know what this means. Yusei Fudo is no hero... well... now that Yusei realizes what the future holds and knowing his strong sense of justice, he'll do all that he can to change it, or at least go with the flow and face whatever lies ahead. Pretty typical, though, that every hero of every Yu-Gi-Oh! series is showing negative colors one way or another. What's it gonna be for Yuma in Yu-Gi-Oh! ZEXAL, I wonder?
...Or so this is all meant to be believed, anyway. As it turns out, Z-one isn't really Yusei. He just copied his DNA as a scientist and went around making everyone believe he was to give his people hope.
As for the unknown man claiming to be from Yliaster... he just may be the scientist Z-one somehow... but we may never know... ever.