Finish Mei's story line, which to even play you need to unlock by finishing Lee's story line at least once by completing all 18 songs with him.
I suggest doing Mei's story on the next hardest difficulty compared to what you did Lee's on so you get better faster. Apart from cut scenes the story is exactly the same, in the same locations and songs in same order. There's nothing special about this, so once you finish the last boss battle which is the song Construct you'll earn this Trophy and get to see the proper ending.