"She left me behind?" said X's avatar sadly.
Also, I went back and refreshed myself! Originally, the illusion was just so X's avatar wouldn't try to grab Melody and run. Y always intended to release Melody once she had X's avatar's power. If X's avatar had actually transferred the power as he was supposed to, and Y hadn't delivered Melody, the deal would have been void and the powers would have returned to X's avatar. But that didn't happen, since X's avatar transferred the powers to I disguised as Melody instead; so, as a retcon, I think that Y planned it that way all along. She still didn't expect it from X's avatar, though, since she hadn't realized he had been reincarnated.