1) Ava beat Luxu. Ava takes over Luxu's body and begins impersonating him. What happened to Luxu's heart is unknown at this point.

2) Ava, in Luxu's body, sends HER OWN body into the future as the True Dandelion. The body in the white sheet. That gives her a vessel in the future.

3) Ava, in Luxu's body, bequeaths the no name and the black box to brain during the Union X finale scene. She trusts Brain.This is part of Ava's plan to go against the master.

4) Ava is the true traitor and the Master knows it. He knows that Luxu will eventually be beat and that Ava will assume his identity. This is why he calls luxu, "traitor Luxu" in the Union X finale. Because technically Luxu is the traitor because of the body, but really it's Ava.

5) I came up with this theory watching the KH 3 Epilogue with the Luxu reveal. Xigbar says in response to Gula asking about Ava, "Ava had her own mission, and she CARRIED it out..." While looking up at the sky. This made me think that Xigbar was being very literal. He was talking about how Ava's mission to plant seeds of light in the future included Ava actually sending herself too (carrying herself, the lifeless body in the white sheet).

6) This also explains why Xigbar didnt know anything about the box. Ava takes it all from him and gives it to Brain to do something with. And no one else knows...

7) Ava's body is subject X. or possibly her body with her heart in it. Someone remembered her and had some vessel, her body likely that ended up in Radiant Garden.