Imagine the theory of this being Ava being true. Yes, she has darkness because of what the Master of Masters did to his own students, making them vessels of 7 ancient darknesses against their will, which could be why she attacked Luxu, whom he knew and didn't bother to warn any of them. How would you react if you heard your own master had made you and your colleagues vessels for darkness? You wouldn't take it well. She did what the other foretellers didn’t; she chose the black coat, and she does have a second role after all. That second role is Sora-related. I see others see it being Invi. I would say no, she's still there with Luxu and the others, he is explaining events to them, he's dealing with his colleagues, who, the last time they saw each other, tried to off each other.

We can imply that's what Luxu told her, just as the Master of Masters told Ava her second role, a role, mind you, we don't get to hear. 

Ava, being the easiest to approach, is evident in Khux.

Unlike Ira, Aced, Invi, and Gula, only she and Luxu completed their original roles while the 4 foretellers failed theirs. 

Oh, look, Childe mentioned from the recent quest.

Childe, upon seeing Lumine: Fight me.

Ava: Sure, you’ve dealt with 13 versions, and a 14th shows up, why not?

Luxu: Yes, punch him 14 times.

The Master of Masters sitting in a chair above watching…

Lupa's Side Quest after doing the main quest.

Buling's Side Quest after the main quest.