I’m calling it now, and you can quote me on this. His brother is suspicious, it would not surprise me if he’s the one who did all of that, and if he’s the real “Master” who saved his sister for whatever reason, but killed the village because they might have sold out Light, or for another reason.
Time travel could also play a role, but remember she said Level 9000 not 9999, he’s 9999 so how would he have reverted levels when that hasn’t even been shown as something that people can do? And then there’s Yume’s story not adding up, her being somewhere different than she remembers. Something really fishy is happening here, but I don’t know what yet. Not enough information to really tell one way or the other.